Ariel Knafo, associate professor of psychology at the Hebrew University of Jerusalem, responds:

The jury is still out on whether we are fundamentally generous or greedy and whether these tendencies are shaped by our genes or environment.

Some evidence points to humans being innately cooperative. Studies show that in the first year of life, infants exhibit empathy toward others in distress. At later stages in life we routinely work together to reach goals and help out in times of need.

Yet instances of selfish behavior also abound in society. One recent study used a version of the classic Prisoner's Dilemma, which can test people's willingness to set aside selfish interests to reach a greater good. After modeling different strategies and outcomes, the researchers found that being selfish was more advantageous than cooperating. The benefit may be short-lived, however. Another study showed that players who cooperated did better in the long run.

It seems that human nature supports both prosocial and selfish traits. Genetic studies have made some progress toward identifying their biological roots. By comparing identical twins, who share nearly 100 percent of their genes, and fraternal twins, who share about half, researchers have found overwhelming evidence for genetic effects on behaviors such as sharing and empathy. In these twin studies, identical and fraternal twins are placed in hypothetical scenarios and asked, for example, to split a sum of money with a peer. Such studies often also rely on careful psychological assessments and DNA analysis.

Other work highlights specific genes as key players. My colleagues and I recently identified a gene linked to altruistic behavior and found that a particular variant of it was associated with more selfish behavior in preschoolers.

As for how we might have acquired a genetic blueprint for collaboration, evolutionary scientists offer several explanations. Cooperative behavior may have evolved first among relatives to promote the continuation of their genetic line. As communities diversified, such mutual support could have broadened to include individuals not linked by blood. Another possibility is that humans cooperate to gain some advantage, such as a boost in reputation. Finally, a hotly debated idea is that evolutionary processes take place at the group level. Groups of highly cooperative individuals have higher chances of survival because they can work together to reach goals that are unattainable to less cooperative groups.

Yet almost no behavior is entirely genetic, even among identical twins. Culture, school and parenting are important determinants of cooperation. Thus, the degree to which we act cooperatively or selfishly is unique to each individual and hinges on a variety of genetic and environmental influences.