In researching for a review of The Nurture Assumption, I read over the debate between Harris and Sulloway over birth order effects on personality. Sulloway's thesis, explained in Born to Rebel, is that last-born children have more rebellious, agreeable, and open-minded/liberal personalities, and that this manifests itself in history with revolutions spearheaded by last-borns. This runs in contrast to Harris's theory that the family environment has no lasting impact on personality, so she spends a good deal of time in her books and articles critiquing it.

The whole debate makes my head dizzy. A seemingly simple empirical question has produced years of arguing over methodology. I'm not going to go over the tedious back and forth here, except to say that you can see what both sides have to say with a Google search.

Large, controlled studies have not been kind to Sulloway's thesis. Freese, Powell, and Steelman (1999) looked for a relationship between birth order (controlled for family size) and a variety of political measures on the nationally representative General Social Survey (GSS). They found no significant associations, contrary to Sulloway's predictions.

I decided to look at the GSS myself, this time to see whether questions that tapped into personality characteristics outside of politics showed any relationship with birth order (SIBORDER), when sibship size (SIBS) was controlled for. I excluded only children. I used the Multiple Regressions feature on the Berkeley SDA tool. I found no significant associations between birth order and any of the four variables I looked at:

• MEMLIT (proxy for openness/creativity)- "Here is a list of various organizations. Could you tell me whether or not you are a member of each type? m. Literary, art, discussion or study groups"


• TRUST (proxy for agreeableness) - "Generally speaking, would you say that most people can be trusted or that you can't be too careful in life?"


• WORLD4 (proxy for agreeableness) - "People have different images of the world and human nature. We'd like to know the kinds of images you have. Here is a card with sets of contrasting images. On a scale of 1-7 where would you place your image of the world and human nature between the two contrasting images? 1. Human nature is basically good. 7. Human nature is fundamentally perverse and corrupt."


• OBEYLAW (proxy for rebelliousness) - "In general, would you say that people should obey the law without exception, or are there exceptional occasions on which people should follow their consciences even if it means breaking the law?"

I wouldn't say that we should write off the idea of birth order influences on personality and intelligence, only that we should be very skeptical of them. To the extent that they do exist, they're probably not very significant.

Labels: Psychology