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Weather and climate on Earth are determined by the amount and distribution of incoming radiation from the sun. For an equilibrium climate, outgoing longwave (infrared) radiation (OLR) necessarily balances the incoming absorbed solar radiation (ASR), so that the Net =ASR-OLR =0. There is a great deal of fascinating atmosphere, ocean and land phenomena that couple the ASR and OLR and the balance is only for the annual mean, not individual months or seasons. Incoming radiant energy may be scattered and reflected by clouds and aerosols, or absorbed in the atmosphere. The transmitted radiation is then either absorbed or reflected at the Earth’s surface. Radiant solar (shortwave) energy is transformed into sensible heat, latent energy (involving different water states), potential energy (involving gravity and height above the surface (or in the oceans, depth below)) and kinetic energy (involving motions) before being emitted back to space as longwave radiant energy. Energy may be stored for some time, transported in various forms, and converted among the different types, giving rise to a rich variety of weather or turbulent phenomena in the atmosphere and ocean. Moreover, the energy balance can be upset in various ways (so the Net ≠ 0), changing the climate and associated weather.

The atmosphere does not have much capability to store heat. The heat capacity of the global atmosphere corresponds to that of only a 3.5 m layer of the ocean. However, the depth of ocean actively involved in climate is much greater than that. The specific heat of dry land is roughly a factor of 4.5 less than that of sea water (for moist land the factor is probably closer to 2). Moreover, heat penetration into land is limited by the low thermal conductivity of the land surface; as a result only the top few meters of the land typically play an active role in heat storage and release (e.g., as the depth for most of the variations over annual time scales). Accordingly, land plays a much smaller role than the ocean in the storage of heat and in providing a memory for the climate system. Major ice sheets over Antarctica and Greenland have a large mass but, like land, the penetration of heat occurs primarily through conduction so that the mass experiencing temperature changes from year to year is small. Hence, ice sheets and glaciers do not play a strong role in global mean heat capacity except on greater than century time scales, while sea ice is important in those places where it forms. Unlike land, however, ice caps and ice sheets melt, altering sea level, albeit fairly slowly.

The oceans cover about 71% of the Earth’s surface and contain 97% of the Earth’s water. Through their fluid motions, their high heat capacity, and their ecosystems, the oceans play a central role in shaping the Earth’s climate and its variability. The seasonal variations in heating penetrate into the ocean through a combination of radiation, convective overturning (in which cooled surface waters sink while warmer, more buoyant waters below rise) and mechanical stirring by winds. These processes mix heat through the mixed layer. Accordingly, it is vital to monitor and understand changes in the oceans and their effects on weather and climate.

The present-day climate is changing mainly in response to human-induced variations in the composition of the atmosphere as increases in greenhouse gases, such as carbon dioxide from burning of fossil fuels, promote warming. In contrast, changes in visible pollution (particulate aerosols) add many complications regionally and can add to or subtract from any warming depending on the nature of the aerosols and their interactions with clouds. The normal flow of energy through the climate system is about 122 PW (1 Petawatt =1015 watts) (see Fig. 2 presented later below). Human activities also contribute directly to local warming through burning of fossil fuels, thereby adding heat, estimated globally to be about 1/9000 (0.01%) of the normal flow of energy (Karl and Trenberth, 2003), while radiative forcing from increased greenhouse gases (IPCC, 2007) is estimated to be about 1.3% (1.6 PW), and the total net anthropogenic radiative forcing once aerosol cooling is factored in is estimated to be about 0.7%. [Radiative forcing is the change without factoring in the effects of the response and feedbacks]. The main negative feedback is from radiation: warming promotes higher temperatures and thus more longwave cooling. The actual imbalance at the top-of-atmosphere (TOA) would increase to about 1.5% once water vapor and ice-albedo feedbacks are included, but the total is reduced and is estimated to be about 0.5 PW (0.4%) owing to the other responses of the climate system; by increasing temperatures, outgoing longwave radiation (OLR) is increased as partial compensation. Unfortunately, these values are too small to yet be directly measured from space, but their consequences can be seen and measured, at least in principle.

Fig. 1. The global annual mean Earth’s energy budget for the March 2000 to May 2004 period in W m-2. The broad arrows indicate the schematic flow of energy in proportion to their importance. From Trenberth et al. (2009).


Figure 2: CERES-period March 2000 to May 2004 mean best-estimate TOA fluxes [PW] globally (center grey) and for globalland (right, light grey) and global-ocean (left) regions. SI is the solar irradiance and the net downward radiation RT =ASROLR. The arrows show the direction of the flow. ∇.FA is the divergence of the atmospheric energy transport and the center arrow indicates the energy flow from ocean to land. The net surface flux is also given. Adapted from Fasullo and Trenberth (2008a).


Understanding and tracking the changes in the flow of energy through the climate system as the climate changes are important for assessments of what is happening to the climate and what the prospects are in the future. Here we comment on our ability to track the energy flow changes.

2. Global mean energy flows

Since about 2000, measurements from instruments on satellite platforms have provided new estimates of global radiation from the Clouds and the Earth’s Radiant Energy System (CERES) instrument. A summary of the overall energy balance for the global atmosphere for the recent period (about 2000 to 2004) (Fig. 1) has the units of Watts per unit area. The global flows in Fig. 1 include reflection by clouds and the surface of solar radiation, and absorption by water vapor and aerosols. The energy balance at the surface is achieved through the incoming solar being mainly compensated by evaporative cooling (which drives the hydrological cycle), longwave radiation, and direct sensible heating. The very large surface longwave emissions are compensated by large back radiation by greenhouse gases and clouds, such that the evaporative cooling is larger as a whole. The global net imbalance is estimated to be 0.9 W m-2.

Fig. 2 shows the flows for the atmosphere in the ocean and land domains. Here the areas are accounted for and the units are Petawatts. Plus and minus twice the standard deviation of the interannual variability is given in the figure as an error bar. The net imbalance in the top of the atmosphere (TOA) radiation is 0.5±0.3 PW (0.9 W -2) out of a net flow through the climate system of about 122 PW of energy (as given by the ASR and OLR). The fossil fuel consumption term is too small to enter into this figure. Hence the imbalance is about 0.4%. Most of this goes into the oceans, and about 0.01 PW goes into land and melting of ice. However, there is an annual mean transport of energy by the atmosphere from ocean to land regions of 2.2±0.1 PW, primarily in the northern winter when the transport exceeds 5 PW.

When all information is combined, there are residuals that indicate errors, which can be traced to ocean heat content in the historical record, and in particular to insufficient or no sampling of the ocean in the southern hemisphere in their winter. This situation has been alleviated since about 2002 when new ARGO floats (see have been deployed that drift freely at a depth of about 2000 m, and about once per 5 days, pop up to the surface using an ingenious small pump to change the float’s volume, making a sounding of temperature and salinity along the way. The soundings are transmitted via satellite to land stations and processed to provide a comprehensive view of the ocean.

Figure 3: Zonal mean meridional energy transport by total (solid), the atmosphere (dashed), and by the ocean (dotted) accompanied with the associated ±2σ range (shaded). Adapted from Fasullo and Trenberth (2008b)

In the tropical ocean, the surface flux of energy is balanced principally by the transport of ocean energy (mainly heat), while in mid-latitudes surface fluxes are largely balanced locally by changes in ocean heat storage. The annual and zonal mean meridional energy transport by the atmosphere and ocean, and their sum (Fig. 3) show that the atmospheric transports dominate except in the tropics. There is a pronounced annual cycle of poleward ocean heat transport into the winter hemisphere exceeding 4 PW in the tropics, but the annual mean value across the equator is near zero. For the annual mean, the poleward transport by the ocean peaks at 11°S at 1.2 PW and 15°N at 1.7 PW.

3. Changes in energy and sea level rise

As noted above, there is a current radiative imbalance at the top-of-the-atmosphere of about 0.9 W -2 owing to increases of greenhouse gases, notably carbon dioxide, in the atmosphere. This has increased from a very small imbalance only 40 years ago when carbon dioxide increases and radiative forcing were less than half of those today. Where is this heat going? Some heat melts glaciers and ice, contributing mass to the ocean which is called eustatic sea level rise. Some heat enters the ocean and increases temperatures and ocean heat content, leading to expansion of the ocean which is called thermosteric sea level rise. Only very small amounts of heat enter the land. Hence the main candidate for a heat sink is the oceans, and sea level rise synthesizes both expansion and added mass from melting of ice elements. Accordingly, it is an excellent indicator of warming.

To be more concrete, a 1 mm rise in sea level requires melting of 360 Gt of ice which takes 1.2×1020 J. Because the ice is cold, warming of the melted waters to ambient temperatures can account for perhaps another 12.5% of the energy (total 1.35×1020 J). Sea level rise from thermal expansion depends greatly on where the heat is deposited as the coefficient of thermal expansion varies with temperature and pressure (the saline ocean does not have a maximum in density at 4°C as fresh water does). The amount of warming required to produce 1 mm sea level rise due to expansion if the heat is deposited in the top 700 m of the ocean can take from 50 to 75×1020 J, or ~110×1020 J if deposited below 700 m depth. Hence melting ice is a factor of about 40 to 70 times more effective than thermal expansion in raising sea level when heat is deposited in upper 700 m; the factor is ~90 when heat is deposited below 700 m depth. For comparison, 0.9 W -2 integrated globally is equivalent to about 1.4×1022 J/yr, which is a sea level equivalent of ~84 mm from ice melt or 1.3 to 2.7 mm from thermosteric ocean expansion. Note however that ice-laden land occupies only a few percent of the globe, which reduces the potential ice melt to only 1 to 2 mm/yr. Accordingly, for sea level rise to relate to energy budgets it is essential to know the eustatic and thermosteric components.

Fig. 4. Global sea level since August 1992. The TOPEX/ Poseidon satellite mission provided observations of sea level change from 1992 until 2005. Jason-1, launched in late 2001 continues this record by providing an estimate of global mean sea level every 10 days with an uncertainty of 3-4 mm. The seasonal cycle has been removed and an atmospheric pressure correction has been applied. http:// Courtesy Steve Nerem (reproduced with permission).

Sea level is estimated to have risen throughout the 20th century by 1.8±0.5 mm/yr. The rate of sea level rise from 1993 to 2007, when accurate satellite-based global measurements of sea level from TOPEX/Poseidon and Jason altimetry are available, average about 3.1 mm/year (Fig. 4). For 1993 to 2003, there is a reasonable accounting for how this comes about. Contributions from glaciers and small ice caps and from the ice sheets of Antarctica and Greenland add mass to the oceans and eustatic rise of about 1.2 mm/yr. Contributions from changes in storage of water on land in reservoirs and dams may account for –0.55 mm/yr sea level equivalent, but these are compensated for by ground water mining, urbanization, and deforestation effects. Direct temperature measurements within the ocean show that ocean heat content increased and sea level rose from thermal expansion by 1.6 to 1.8 mm/yr. About 0.3 mm/yr is from slow isostatic rebound of the Earth’s crust.

Since 2003, however, when ARGO floats have provided better data, increase in ocean heat content has slowed, while Greenland and Antarctica melting has picked up. Whether or not the sea level budget is closed, it is not clear that the global energy budget is closed because sea level rise is much greater for land ice melt versus ocean expansion for a given amount of heat, as noted above. Accordingly, another much needed component is the TOA radiation, but CERES data are not yet processed beyond 2004 and are not yet long enough to bring to bear on this question.

4. Climate Change

A consequence of the energy imbalance at the TOA is global warming. In 2007 the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), known as AR4, clearly stated that “warming of the climate system is unequivocal” and it is “very likely” due to human activities. Since the IPCC report, nature continues to provide evidence that it is under duress with impacts affecting people and animals. Increasing rates of carbon dioxide emissions raise the specter that future climate changes could be much larger and come much quicker than IPCC suggests.

The AR4 found that warming of the climate system is unequivocal based on an increasing body of evidence showing discernible physically consistent changes. These include increases in global average surface air temperature; atmospheric temperatures above the surface, surface and sub-surface ocean water temperature; widespread melting of snow; decreases in Arctic sea-ice extent and thickness; decreases in glacier and small ice cap extent and mass; and rising global mean sea level. The observed surface warming at global and continental scales is also consistent with reduced duration of freeze seasons; increased heat waves; increased atmospheric water vapor content and heavier precipitation events; changes in patterns of precipitation; increased drought; increases in intensity of hurricane activity, and changes in atmospheric winds. This wide variety of observations gives a very high degree of confidence to the overall findings. Because these changes are now simulated in climate models for the past 100 years to a reasonable degree, there is added confidence in future projections for more warming and increased impacts. Moreover, these changes in physical variables are reflected in changes in ecosystems and human health.

Carbon dioxide concentrations are increasing at rates beyond the highest of the IPCC scenarios, suggesting even bigger and faster climate change than IPCC projected. Warming is manifested in multiple ways, not just increases in temperatures. Most dramatic is the loss of Arctic sea ice in 2007 and 2008, which affects surrounding areas, polar bears and other native species and promotes changes in permafrost. Distinctive patterns of temperature and precipitation anomalies in the winter of 2007-08 were characteristic of the strong La Niña that had a signature over most of the world. In the first 6 months of 2008, record heavy rains and flooding in Iowa, Ohio, and Missouri, led to overtopped levees along the Cedar River in Iowa and the Mississippi, and point to increases in intensity of rains associated with more water vapor in the atmosphere: a direct consequence of warming. The record-breaking numbers of tornadoes and deaths in the U.S. in 2008 probably also have a global warming component from the warm moist air coming out of the Gulf of Mexico adding to instability of the atmosphere. Longer dry spells also accompany warming, as heat goes into evaporating moisture, drying and wilting vegetation, and thus increasing the risk of wild fire enormously. Wild fires in California early in 2008 and again last summer are evidence of the impacts. Hurricanes are becoming more active. In the Atlantic in July 2008, hurricane Bertha broke several records for how early and how far east it formed, and it is the longest lasting July hurricane. Fay made landfall 4 times and hurricanes Gustav and Ike caused devastation in the U.S. in 2008. Sea level rise continues at a rate of over a foot a century. Changes in ocean acidity accompany the buildup in carbon dioxide in the atmosphere with consequences for sea creatures, and bleaching of corals occurs in association with warming oceans. Melting permafrost exposes huge potential sources of methane and carbon dioxide that can amplify future climate change. Global warming is not just a threat for the future, it is already happening, endangering the health and welfare of the planet. There is a crisis of inaction in addressing and preparing for climate change.

Dr. Kevin E. Trenberth is Head of the Climate Analysis Section at the National Center for Atmospheric Research. He was a lead author of the 1995, 2001 and 2007 Intergovernmental Panel on Climate Change (IPCC) Scientific Assessment of Climate Change


Fasullo, J.T., K.E. Trenberth, 2008a: The annual cycle of the energy budget: Pt I. Global mean and land-ocean exchanges. J. Climate 21, 2297−2313.

Fasullo, J.T., K.E. Trenberth, 2008b: The annual cycle of the energy budget: Pt II. Meridional structures and poleward transports. J. Climate 21, 2314−2326.

IPCC (Intergovernmental Panel on Climate Change), 2007: Climate Change 2007: The Physical Science Basis. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor, H.L. Miller (Eds.). Cambridge University Press, Cambridge, UK. 996 pp.

Karl, T. R., K.E. Trenberth, 2003: Modern global climate change. Science 302, 1719–1723.

Trenberth, K.E., J.T. Fasullo, and J. Kiehl, 2009: Earth’s global energy budget. Bull. Amer. Meteor. Soc., doi:10.1175/2008BAMS2634.1. (in press).

This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.


he Impact of Electronic Media Violence: Scientific Theory and Research

L. Rowell Huesmann

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The publisher's final edited version of this article is available at J Adolesc Health

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Since the early 1960s research evidence has been accumulating that suggests that exposure to violence in television, movies, video games, cell phones, and on the internet increases the risk of violent behavior on the viewer’s part just as growing up in an environment filled with real violence increases the risk of them behaving violently. In the current review this research evidence is critically assessed, and the psychological theory that explains why exposure to violence has detrimental effects for both the short run and long run is elaborated. Finally, the size of the “media violence effect” is compared with some other well known threats to society to estimate how important a threat it should be considered.

One of the notable changes in our social environment in the 20th and 21st centuries has been the saturation of our culture and daily lives by the mass media. In this new environment radio, television, movies, videos, video games, cell phones, and computer networks have assumed central roles in our children’s daily lives. For better or worse the mass media are having an enormous impact on our children’s values, beliefs, and behaviors. Unfortunately, the consequences of one particular common element of the electronic mass media has a particularly detrimental effect on children’s well being. Research evidence has accumulated over the past half-century that exposure to violence on television, movies, and most recently in video games increases the risk of violent behavior on the viewer’s part just as growing up in an environment filled with real violence increases the risk of violent behavior. Correspondingly, the recent increase in the use of mobile phones, text messaging, e-mail, and chat rooms by our youth have opened new venues for social interaction in which aggression can occur and youth can be victimized – new venues that break the old boundaries of family, neighborhood, and community that might have protected our youth to some extent in the past. These globe spanning electronic communication media have not really introduced new psychological threats to our children, but they have made it much harder to protect youth from the threats and have exposed many more of them to threats that only a few might have experienced before. It is now not just kids in bad neighborhoods or with bad friends who are likely to be exposed to bad things when they go out on the street. A ‘virtual’ bad street is easily available to most youth now. However, our response should not be to panic and keep our children “indoors” because the “streets” out there are dangerous. The streets also provide wonderful experiences and help youth become the kinds of adults we desire. Rather our response should be to understand the dangers on the streets, to help our children understand and avoid the dangers, to avoid exaggerating the dangers which will destroy our credibility, and also to try to control exposure to the extent we can.

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Background for the Review

Different people may have quite different things in mind when they think of media violence. Similarly, among the public there may be little consensus on what constitutes aggressive and violent behavior. Most researchers, however, have clear conceptions of what they mean by media violence and aggressive behavior.

Most researchers define media violence as visual portrayals of acts of physical aggression by one human or human-like character against another. This definition has evolved as theories about the effects of media violence have evolved and represents an attempt to describe the kind of violent media presentation that is most likely to teach the viewer to be more violent. Movies depicting violence of this type were frequent 75 years ago and are even more frequent today, e.g., M, The Maltese Falcon, Shane, Dirty Harry, Pulp Fiction, Natural Born Killers, Kill Bill. Violent TV programs became common shortly after TV became common in American homes about 55 years ago and are common today, e.g., Gunsmoke, Miami Vice, CSI, and 24. More recently, video games, internet displays, and cell phone displays have become part of most children’s growing-up, and violent displays have become common on them, e.g., Grand Theft Auto, Resident Evil, Warrior.

To most researchers, aggressive behavior refers to an act that is intended to injure or irritate another person. Laymen may call assertive salesmen “aggressive,” but researchers do not because there is no intent to harm. Aggression can be physical or non-physical. It includes many kinds of behavior that do not seem to fit the commonly understood meaning of “violence.” Insults and spreading harmful rumors fit the definition. Of course, the aggressive behaviors of greatest concern clearly involve physical aggression ranging in severity from pushing or shoving, to fighting, to serious assaults and homicide. In this review he term violent behavior is used to describe these more serious forms of physical aggression that have a significant risk of seriously injuring the victim.

Violent or aggressive actions seldom result from a single cause; rather, multiple factors converging over time contribute to such behavior. Accordingly, the influence of the violent mass media is best viewed as one of the many potential factors that influence the risk for violence and aggression. No reputable researcher is suggesting that media violence is “the” cause of violent behavior. Furthermore, a developmental perspective is essential for an adequate understanding of how media violence affects youthful conduct and in order to formulate a coherent response to this problem. Most youth who are aggressive and engage in some forms of antisocial behavior do not go on to become violent teens and adults [1]. Still, research has shown that a significant proportion of aggressive children are likely to grow up to be aggressive adults, and that seriously violent adolescents and adults often were highly aggressive and even violent as children [2]. The best single predictor of violent behavior in older adolescents, young adults, and even middle aged adults is aggressive behavior when they were younger. Thus, anything that promotes aggressive behavior in young children statistically is a risk factor for violent behavior in adults as well.

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Theoretical Explanations for Media Violence Effects

In order to understand the empirical research implicating violence in electronic media as a threat to society, an understanding of why and how violent media cause aggression is vital. In fact, psychological theories that explain why media violence is such a threat are now well established. Furthermore, these theories also explain why the observation of violence in the real world – among the family, among peers, and within the community – also stimulates aggressive behavior in the observer.

Somewhat different processes seem to cause short term effects of violent content and long term effects of violent content, and that both of these processes are distinct from the time displacement effects that engagement in media may have on children. Time displacement effects refer to the role of the mass media (including video games) in displacing other activities in which the child might engage which might change the risk for certain kinds of behavior, e.g. replacing reading, athletics, etc. This essay is focusing on the effects of violent media content, and displacement effects will not be reviewed though they may well have important consequences.

Short-term Effects

Most theorists would now agree that the short term effects of exposure to media violence are mostly due to 1) priming processes, 2) arousal processes, and 3) the immediate mimicking of specific behaviors [3, 4].



Priming is the process through which spreading activation in the brain’s neural network from the locus representing an external observed stimulus excites another brain node representing a cognition, emotion, or behavior. The external stimulus can be inherently linked to a cognition, e.g., the sight of a gun is inherently linked to the concept of aggression [5], or the external stimulus can be something inherently neutral like a particular ethnic group (e.g., African-American) that has become linked in the past to certain beliefs or behaviors (e.g., welfare). The primed concepts make behaviors linked to them more likely. When media violence primes aggressive concepts, aggression is more likely.



To the extent that mass media presentations arouse the observer, aggressive behavior may also become more likely in the short run for two possible reasons -- excitation transfer [6] and general arousal [7]. First, a subsequent stimulus that arouses an emotion (e.g. a provocation arousing anger) may be perceived as more severe than it is because some of the emotional response stimulated by the media presentation is miss-attributed as due to the provocation transfer. For example, immediately following an exciting media presentation, such excitation transfer could cause more aggressive responses to provocation. Alternatively, the increased general arousal stimulated by the media presentation may simply reach such a peak that inhibition of inappropriate responses is diminished, and dominant learned responses are displayed in social problem solving, e.g. direct instrumental aggression.



The third short term process, imitation of specific behaviors, can be viewed as a special case of the more general long-term process of observational learning [8]. In recent years evidence has accumulated that human and primate young have an innate tendency to mimic whomever they observe [9]. Observation of specific social behaviors around them increases the likelihood of children behaving exactly that way. Specifically, as children observe violent behavior, they are prone to mimic it. The neurological process through which this happens is not completely understood, but it seems likely that “mirror neurons,” which fire when either a behavior is observed or when the same behavior is acted out, play an important role [10, 4].

Long-term Effects

Long term content effects, on the other hand, seem to be due to 1) more lasting observational learning of cognitions and behaviors (i.e., imitation of behaviors), and 2) activation and desensitization of emotional processes.


Observational learning

According to widely accepted social cognitive models, a person’s social behavior is controlled to a great extent by the interplay of the current situation with the person’s emotional state, their schemas about the world, their normative beliefs about what is appropriate, and the scripts for social behavior that they have learned [11]. During early, middle, and late childhood children encode in memory social scripts to guide behavior though observation of family, peers, community, and mass media. Consequently observed behaviors are imitated long after they are observed [10]. During this period, children’s social cognitive schemas about the world around them also are elaborated. For example, extensive observation of violence has been shown to bias children’s world schemas toward attributing hostility to others’ actions. Such attributions in turn increase the likelihood of children behaving aggressively [12]. As children mature further, normative beliefs about what social behaviors are appropriate become crystallized and begin to act as filters to limit inappropriate social behaviors [13]. These normative beliefs are influenced in part by children’s observation of the behaviors of those around them including those observed in the mass media.



Long-term socialization effects of the mass media are also quite likely increased by the way the mass media and video games affect emotions. Repeated exposures to emotionally activating media or video games can lead to habituation of certain natural emotional reactions. This process is called “desensitization.” Negative emotions experienced automatically by viewers in response to a particular violent or gory scene decline in intensity after many exposures [4]. For example, increased heart rates, perspiration, and self-reports of discomfort often accompany exposure to blood and gore. However, with repeated exposures, this negative emotional response habituates, and the child becomes “desensitized.” The child can then think about and plan proactive aggressive acts without experiencing negative affect [4].


Enactive learning

One more theoretical point is important. Observational learning and desensitization do not occur independently of other learning processes. Children are constantly being conditioned and reinforced to behave in certain ways, and this learning may occur during media interactions. For example, because players of violent video games are not just observers but also “active” participants in violent actions, and are generally reinforced for using violence to gain desired goals, the effects on stimulating long-term increases in violent behavior should be even greater for video games than for TV, movies, or internet displays of violence. At the same time, because some video games are played together by social groups (e.g., multi-person games) and because individual games may often be played together by peers, more complex social conditioning processes may be involved that have not yet been empirically examined. These effects, including effects of selection and involvement, need to be explored.

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The Key Empirical Studies

Given this theoretical back ground, let us now examine the empirical research that indicates that childhood exposure to media violence has both short term and long term effects in stimulating aggression and violence in the viewer. Most of this research is on TV, movies, and video games, but from the theory above one can see that the same effects should occur for violence portrayed on various internet sites (e.g., multi-person game sites, video posting sites, chat rooms) and on handheld cell phones or computers.

Violence in Television, Films, and Video Games

The fact that most research on the impact of media violence on aggressive behavior has focused on violence in fictional television and film and video games is not surprising given the prominence of violent content in these media and the prominence of these media in children’s lives.

Children in the United States spend an average of between three and four hours per day viewing television [14], and the best studies have shown that over 60% of programs contain some violence, and about 40% of those contain heavy violence [15]. Children are also spending an increasingly large amount of time playing video games, most of which contain violence. Video game units are now present in 83% of homes with children [16]. In 2004, children spent 49 minutes per day playing video, and on any given day, 52% of children ages 8–18 years play a video game games [16]. Video game use peaks during middle childhood with an average of 65 minutes per day for 8–10 year-olds, and declines to 33 minutes per day for 15–18 year-olds [16]. And most of these games are violent; 94% of games rated (by the video game industry) as appropriate for teens are described as containing violence, and ratings by independent researchers suggest that the real percentage may be even higher [17]. No published study has quantified the violence in games rated ‘M’ for mature—presumably, these are even more likely to be violent.

Meta-analyses that average the effects observed in many studies provide the best overall estimates of the effects of media violence. Two particularly notable meta-analyses are those of Paik and Comstock [18] and Anderson and Bushman [19]. The Paik and Comstock meta-analysis focused on violent TV and films while the Anderson and Bushman meta-analysis focused on violent video games.

Paik and Comstock [18] examined effect sizes from 217 studies published between 1957 and 1990. For the randomized experiments they reviewed, Paik and Comstock found an average effect size (r =.38, N=432 independent tests of hypotheses) which is moderate to large compared to other public health effects. When the analysis was limited to experiments on physical violence against a person, the average r was still .32 (N=71 independent tests). This meta-analysis also examined cross-sectional and longitudinal field surveys published between 1957 and 1990. For these studies the authors found an average r of .19 (N=410 independent tests). When only studies were used for which the dependent measure was actual physical aggression against another person (N=200), the effect size remained unchanged. Finally, the average correlation of media violence exposure with engaging in criminal violence was .13.

Anderson and Bushman [19] conducted the key meta-analyses on the effects of violent video games. Their meta-analyses revealed effect sizes for violent video games ranging from .15 to .30. Specifically, playing violent video games was related to increases in aggressive behavior (r = .27), aggressive affect (r =.19), aggressive cognitions (i.e., aggressive thoughts, beliefs, and attitudes), (r =.27), and physiological arousal (r = .22) and was related to decreases in prosocial (helping) behavior (r = −.27). Furthermore, when studies were coded for the quality of their methodology, the best studies yielded larger effect sizes than the “not-best” studies.

One criticism sometimes leveled at meta-analyses is based on the “file drawer effect.” This refers to the fact that studies with “non-significant” results are less likely to be published and to appear in meta-analyses. However, one can correct for this problem by estimating how many “null-effect” studies it would take to change the results of the meta-analysis. This has been done with the above meta-analyses, and the numbers are very large. For example, Paik and Comstock [18] show that over 500,000 cases of null effects would have to exist in file drawers to change their overall conclusion of a significant positive relation between exposure to media violence and aggression.

While meta-analyses are good of obtaining a summary view of what the research shows, a better understanding of the research can be obtained by examining a few key specific studies in more detail.



Generally, experiments have demonstrated that exposing people, especially children and youth, to violent behavior on film and TV increases the likelihood that they will behave aggressively immediately afterwards. In the typical paradigm, randomly selected individuals are shown either a violent or non-violent short film or TV program or play a violent or non-violent video game and are then observed as they have the opportunity to aggress. For children, this generally means playing with other children in situations that might stimulate conflict; for adults, it generally means participating in a competitive activity in which winning seems to involve inflicting pain on another person.

Children in such experiments who see the violent film clip or play the violent game typically behave more aggressively immediately afterwards than those viewing or playing nonviolence (20, 21, 22). For example, Josephson (22) randomly assigned 396 seven- to nine-year-old boys to watch either a violent or a nonviolent film before they played a game of floor hockey in school. Observers who did not know what movie any boy had seen recorded the number of times each boy physically attacked another boy during the game. Physical attack was defined to include hitting, elbowing, or shoving another player to the floor, as well as tripping, kneeing, and other assaultive behaviors that would be penalized in hockey. For some children, the referees carried a walkie-talkie, a specific cue that had appeared in the violent film that was expected to remind the boys of the movie they had seen earlier. For boys rated by their teacher as frequently aggressive, the combination of seeing a violent film and seeing the movie-associated cue stimulated significantly more assaultive behavior than any other combination of film and cue. Parallel results have been found in randomized experiments for preschoolers who physically attack each other more often after watching violent videos [21] and for older delinquent adolescents who get into more fights on days they see more violent films [23].

In a randomized experiment with violent video games, Irwin & Gross [24] assessed physical aggression (e.g., hitting, shoving, pinching, kicking) between boys who had just played either a violent or a nonviolent video game. Those who had played the violent video game were more physically aggressive toward peers. Other randomized experiments have measured college students’ propensity to be physically aggressive after they had played (or not played) a violent video game. For example, Bartholow &Anderson [25] found that male and female college students who had played a violent game subsequently delivered more than two and a half times as many high-intensity punishments to a peer as those who played a nonviolent video game. Other experiments have shown that it is the violence in video games, not the excitement that playing them provokes, that produces the increase in aggression [26].

In summary, experiments unambiguously show that viewing violent videos, films, cartoons, or TV dramas or playing violent video games “cause” the risk to go up that the observing child will behave seriously aggressively toward others immediately afterwards. This is true of preschoolers, elementary school children, high school children, college students, and adults. Those who watch the violent clips tend to behave more aggressively than those who view non-violent clips, and they adopt beliefs that are more “accepting” of violence [27].

One more quasi-experiment frequently cited by game manufacturers should be mentioned here. Williams and Skoric [28] have published the results of a dissertation study of cooperative online game playing by adults in which they report no significant long-term effects of playing a violent game on the adult’s behavior. However, the low statistical power of the study, the numerous methodological flaws (self-selection of a biased sample, lack of an adequate control group, the lack of adequate behavioral measures) make the validity of the study highly questionable. Furthermore, the participants were adults for whom there would be little theoretical reason to expect long-term effects.


Cross-sectional and longitudinal studies

Empirical cross-sectional and longitudinal studies of youth behaving and watching or playing violent media in their natural environments do not test causation as well as experiments do, but they provide strong evidence that the causal processes demonstrated in experiments generalize to violence observed in the real world and have significant effects on real world violent behavior. As reported in the discussion of meta-analyses above, the great majority of competently done one-shot survey studies have shown that children who watch more media violence day in and day out behave more aggressively day in and day out [18]. The relationship is less strong than that observed in laboratory experiments, but it is nonetheless large enough to be socially significant; the correlations obtained are usually are between .15 and .30. Moreover, the relation is highly replicable even across researchers who disagree about the reasons for the relationship [e.g., 29] and across countries [30, 31].

Complementing these one-time survey studies are the longitudinal real-world studies that have shown correlations over time from childhood viewing of media violence to later adolescent and adult aggressive behavior [31, 32, 33, 34, 35]; for reviews see [4, 27, 33]. This studies have shown that early habitual exposure to media violence in middle-childhood predicts increased aggressiveness 1 year, 3 years, 10 years, 15 years, and 22 years later in adulthood, even controlling for early aggressiveness. On the other hand, behaving aggressively in childhood is a much weaker predictor of higher subsequent viewing of violence when initial violence viewing is controlled, making it implausible that the correlation between aggression and violent media use was primarily due to aggressive children turning to watching more violence [31, 32, 33]. As discussed below the pattern of results suggests that the strongest contribution to the correlation is the stimulation of aggression from exposure to media violence but that those behaving aggressively may also have a tendency to turn to watching more violence, leading to a downward spiral effect [13].

An example is illustrative. In a study of children interviewed each year for three years as they moved through middle childhood, Huesmann et al. [31] found increasing rates of aggression for both boys and girls who watched more television violence even with controls for initial aggressiveness and many other background factors. Children who identified with the portrayed aggressor and those who perceived the violence as realistic were especially likely to show these observational learning effects. A 15-year follow-up of these children [33] demonstrated that those who habitually watched more TV violence in their middle-childhood years grew up to be more aggressive young adults. For example, among children who were in the upper quartile on violence viewing in middle childhood, 11% of the males had been convicted of a crime (compared with 3% for other males), 42% had “pushed, grabbed, or shoved their spouse” in the past year (compared with 22% of other males), and 69% had “shoved a person” when made angry in the past year (compared with 50% of other males). For females, 39% of the high-violence-viewers had “thrown something at their spouse” in the past year (compared with 17% of the other females), and 17% had “punched, beaten, or choked” another adult when angry in the past year (compared with 4% of the other females). These effects were not attributable to any of a large set of child and parent characteristics including demographic factors, intelligence, parenting practices. Overall, for both males and females the effect of middle-childhood violence viewing on young adult aggression was significant even when controlling for their initial aggression. In contrast, the effect of middle-childhood aggression on adult violence viewing when controlling for initial violence viewing was not-significant, though it was positive.

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Moderators of Media Violence Effects

Obviously, not all observers of violence are affected equally by what they observe at all times. Research has shown that the effects of media violence on children are moderated by situational characteristics of the presentation including how well it attracts and sustains attention, personal characteristics of the viewer including their aggressive predispositions, and characteristics of the physical and human context in which the children are exposed to violence.

In terms of plot characteristics, portraying violence as justified and showing rewards (or at least not showing punishments) for violence increase the effects that media violence has in stimulating aggression, particularly in the long run [27, 36, 37]. As for viewer characteristics that depend on perceptions of the plot, those viewers who perceive the violence as telling about life more like it really is and who identify more with the perpetrator of the violence are also stimulated more toward violent behavior in the long run [27, 30, 33, 38]. Taken together these facts mean that violent acts by charismatic heroes, that appear justified and are rewarded, are the violent acts most likely to increase viewer’s aggression.

A number of researchers have suggested that, independently of the plot, viewers or game players who are already aggressive should be the only one’s affected. This is certainly not true. While the already aggressive child who watches or plays a lot of violent media may become the most aggressive young adult, the research shows that even initially unaggressive children are made more aggressive by viewing media violence [27, 32, 33]. Long term effects due appear to be stronger for younger children [3, 14], but short term affects appear, if anything, stronger for older children [3] perhaps because one needs to have already learned aggressive scripts to have them primed by violent displays. While the effects appeared weaker for female 40 years ago [32], they appear equally strong today [33]. Finally, having a high IQ does not seem to protect a child against being influenced [27].

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Mediators of Media Violence Effects

Most researchers believe that the long term effects of media violence depend on social cognitions that control social behavior being changed for the long run. More research needs to completed to identify all the mediators, but it seems clear that they include normative beliefs about what kinds of social behaviors are OK [4, 13, 27], world schemas that lead to hostile or non-hostile attributions about others intentions [4, 12, 27], and social scripts that automatically control social behavior once they are well learned [4, 11, 27].

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This review marshals evidence that compelling points to the conclusion that media violence increases the risk significantly that a viewer or game player will behave more violently in the short run and in the long run. Randomized experiments demonstrate conclusively that exposure to media violence immediately increases the likelihood of aggressive behavior for children and adults in the short run. The most important underlying process for this effect is probably priming though mimicry and increased arousal also play important roles. The evidence from longitudinal field studies is also compelling that children’s exposure to violent electronic media including violent games leads to long-term increases in their risk for behaving aggressively and violently. These long-term effects are a consequence of the powerful observational learning and desensitization processes that neuroscientists and psychologists now understand occur automatically in the human child. Children automatically acquire scripts for the behaviors they observe around them in real life or in the media along with emotional reactions and social cognitions that support those behaviors. Social comparison processes also lead children to seek out others who behave similarly aggressively in the media or in real life leading to a downward spiral process that increases risk for violent behavior.

One valid remaining question is whether the size of this effect is large enough that one should consider it to be a public health threat. The answer seems to be “yes.” Two calculations support this conclusion. First, according to the best meta-analyses [18, 19] the long term size of the effect of exposure to media violence in childhood on later aggressive or violent behavior is about equivalent to a correlation of .20 to .30. While some might argue that this explains only 4% to 9% of the individual variation in aggressive behavior, as several scholars have pointed out [39, 40], percent variance explained is not a good statistic to use when predicting low probability events with high social costs. For example, a correlation of 0.3 with aggression translates into a change in the odds of aggression from 50/50 to 65/35 -- not a trivial change when one is dealing with life threatening behavior[40].

Secondly, the effect size of media violence is the same or larger than the effect size of many other recognized threats to public health. In Figure 1 from Bushman and Huesmann [41], the effect sizes for many common threats to public health are compared with the effect that media violence has on aggression. The only effect slightly larger than the effect of media violence on aggression is that of cigarette smoking on lung cancer.


Figure 1

The Relative Strength of Known Public Health Threats.

In summary, exposure to electronic media violence increases the risk of children and adults behaving aggressively in the short-run and of children behaving aggressively in the long-run. It increases the risk significantly, and it increases it as much as many other factors that are considered public health threats. As with many other public health threats, not every child who is exposed to this threat will acquire the affliction of violent behavior, and many will acquire the affliction who are not exposed to the threat. However, that does not diminish the need to address the threat.

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