2026年6月英语四级（第1套 · 含听力）仔细阅读 Passage 2

It is still controversial whether we are fundamentally generous or selfish and whether these tendencies are shaped by our genes or environment.
Some evidence points to humans being born cooperative. At later stages in life we routinely work together to reach goals and help out in times of need.
Yet instances of selfish behavior are also plenty. One recent study tested people's willingness to set aside selfish interests to reach a greater good. It 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 pro-social and selfish traits (特征). Genetic studies have made some progress toward identifying their biological roots. By comparing identical twins, who share nearly 100 percent of their genes, and non-identical twins, who share about half, researchers have found overwhelming evidence for genetic effects on behaviors. In these twin studies, identical and non-identical twins are 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 (利他的) behavior and found that a particular variant of it was associated with more selfish behavior in preschoolers.
According to evolutionary scientists, cooperative behavior may have evolved first among relatives to promote the continuation of their genetic line. As communities diversified, mutual support could have broadened to include individuals not linked by blood. Another possibility is that humans cooperate to gain some advantage, such as boosting reputation. Finally, 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, schooling and parenting are important determinants of cooperation. Thus, the degree to which we act cooperatively or selfishly is unique to each individual and depends on a variety of genetic and environmental influences.