Co-author Scott F. Stoltenberg, a researcher at the University of Nebraska-Lincoln's Behavior Genetics Laboratory, says the findings build on previous studies that suggest a link between relative levels of anxiety and prosocial behavior.
"It makes sense that people who have less social anxiety are more likely to help out," Stoltenberg explained. "When they're confronted with a situation where another person needs help, they don't have a problem going over to them and engaging." A person with social anxiety, in contrast, might experience so much discomfort that he or she would avoid the encounter.
Both serotonin and dopamine are neurotransmitters related to the sensations of pleasure and satisfaction, which may explain why people who perform selfless acts of generosity report that they feel good as a result.
Carlo cautioned that the study's findings don't necessarily mean that people with a genetic predisposition toward anxiety also lack empathy, the ability to care about others. While it may be more difficult for them to engage in public acts of prosocial behavior, they may instead make anonymous contributions to a person in need, or help in some other way that doesn't require personal interaction.
Why humans developed the capacity to be Good Samaritans is another widely-debated question. In the 1970s, evolutionary biologist Richard Dawkins, a believer in the notion that "genes are selfish," argued that prosocial behavior existed to ensure genetic continuity. His view was influenced by studies suggesting that organisms were most likely to help their own kin.
But as Ohio State University psychologists Baldwin M. Way and Kyle G. Ratner wrote in an essay that appeared in the same journal as Carlo's and Stoltenberg's study, Dawkins' view fails to account for the many instances in which humans have helped others to whom they were not closely related, and have done so with no apparent genetic benefit to themselves.
Previous studies have indicated that the tendency toward prosocial behavior may be at least in part heritable -- that is, passed on from generation to generation genetically -- rather than totally the result of the moral influence of parents or teachers. A 2007 study of Korean twins, for example, found that about 55 percent of the variance in prosocial behavior seemed to be due to genetics, and that the genetic link seemed to increase as the children got older.