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Older siblings really don't share: birth order stereotypes found to be valid

Scientists have determined that birth-order stereotypes are valid, and that eldest children really are less cooperative, trusting, and reciprocating than their siblings.
Written by Andrew Nusca, Contributor on

Scientists have determined that birth-order stereotypes are valid, and that eldest children really are less cooperative, trusting, and reciprocating than their siblings.

For decades, birth order stereotypes have dictated that people's personalities: firstborn children are overachievers, middleborn children are more social because they get the least amount of attention from their parents, and so forth.

Now, a team of researchers led by evolutionary biologist Alexandre Courtiol of the University of Montpellier 2 in France says the stereotypes are true.

In a study, the researchers asked 510 unrelated college students to play a two-person investment game. The game worked like so:

  • Both players start with €3, or approx. $4.50.
  • Player A, the investor, can send any amount of his money to player B, the banker, who could triple that money.
  • Then player B can return any amount of his now larger pool of cash to player A.
  • Because player B doesn't have to send any money back, the amount player A sends to him is a measure of trust. The sum player B returns to player A is a measure of reciprocity.

The scientists randomly assigned each volunteer to play the role of player A or B and told them that they would not meet their partner. (Unbeknownst to them, they were actually playing fictitious opponents.)

By grouping the game data by birth order, the researchers found that firstborn player As trusted less than laterborns, sending 25 percent less money to player B.

Firstborn player Bs also reciprocated less, returning between 22 percent and 29 percent less.

The researchers found that birth order was a stronger factor in players' cooperative behavior than age, gender, income level, or religious belief.

Why the difference? Family dynamics based on birth order, the scientists found.

For example, firstborns are less cooperative because they feel the need to compete more and cooperate less when new siblings arrive and draw away parental attention.

The researchers said the behavior of the only child fits that explanation: they act like middle and lastborns.

In other words: it's not nature, it's nurture, and behavior only changes when another sibling arrives.

One caveat, however: despite the influence of birth order on behavior, it explains less than 10 percent of the variation in subjects' game behavior, the researchers said.

Their findings are published in the December issue of Animal Behavior.

This post was originally published on Smartplanet.com

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