If you watch enough television, you'll witness what psychologists describe as birth order stereotypes. Take Alex P. Keaton of the 1980s U.S. sitcom Family Ties. Firstborn Alex was far more brash and competitive than his younger sisters, reading The Wall Street Journal while in high school, for example. Now scientists report that the stereotype is valid: eldest children are less cooperative, trusting, and reciprocating than their siblings.
Psychologists have been debating the importance of birth order since the days of Sigmund Freud. Those that argue that it plays a strong role in personality say, for instance, that middleborn children are more social than their youngest or oldest siblings because they get the least amount of attention from their parents and thus must make friends outside of their family. Psychologists base their findings on self-questionnaires and interviews with friends and family.
Evolutionary biologist Alexandre Courtiol of the University of Montpellier 2 in France and colleagues wanted a more objective test. So they asked 510 unrelated college students to play a two-person investment game. The game worked like this: Both players started with €3. Player A, the investor, could send any amount of her money to player B, the banker, who would triple that money. Then player B could return any amount of his now larger pool of cash to player A. Because player B didn't have to send any money back, the amount player A sends to him is a measure of trust. And the sum player B returns to player A is therefore a measure of reciprocity.