version of the ecological model is often used in the study of violence: The first level identifies biological and personal factors that influence how individuals behave and increase their likelihood of becoming a victim or perpetrator of violence: demographic characteristics (age, education, income), genetics, brain lesions, personality disorders, substance abuse, and a history of experiencing, witnessing, or engaging in violent behaviour. The second level focuses on close relationships, such as those with family and friends. In youth violence, for example, having friends who engage in or encourage violence can increase a young person's risk of being a victim or perpetrator of violence. For intimate partner violence, a consistent marker at this level of the model is marital conflict or discord in the relationship. In elder abuse, important factors are stress due to the nature of the past relationship between the abused person and the care giver. The third level explores the community context—i.e., schools, workplaces, and neighbourhoods. Risk at this level may be affected by factors such as the existence of a local drug trade, the absence of social networks, and concentrated poverty. All these factors have been shown to be important in several types of violence. Finally, the fourth level looks at the broad societal factors that help to create a climate in which violence is encouraged or inhibited: the responsiveness of the criminal justice system, social and cultural norms regarding gender roles or parent-child relationships, income inequality, the strength of the social welfare system, the social acceptability of violence, the availability of weapons, the exposure to violence in mass media, and political instability. Child-rearing While studies showing associations between physical punishment of children and later aggression cannot prove that physical punishment causes an increase in aggression, a number of longitudinal studies suggest that the experience of physical punishment has a direct causal effect on later aggressive behaviors. Cross-cultural studies have shown that greater prevalence of corporal punishment of children tends to predict higher levels of violence in societies. For instance, a 2005 analysis of 186 pre-industrial societies found that corporal punishment was more prevalent in societies which also had higher rates of homicide, assault, and war. In the United States, domestic corporal punishment has been linked to later violent acts against family members and spouses. The American family violence researcher Murray A. Straus believes that disciplinary spanking forms "the most prevalent and important form of violence in American