The study partially summarized below is of considerable interest in its own right but has broader implications as well. It does of course once more show that environmental effects on intelligence which were once thought important are not in fact important but, as well as that, it shows large holes in the research procedures of previous birth-order studies generally. The notorious Frank Sulloway is of course known for his frantic pursuit of the quite absurd claim that birth-order determines your politics. According to Sulloway, my younger brother should be a leftist -- because he is a younger brother. Like another younger brother among my relatives, however, my younger brother is "so far Right he is almost out of sight". But Sulloway did not take all the precautions in his research that have been shown as important below so it is no surprise on those grounds alone that he came to strange conclusions. ALL the existing birth-order research must now be considered to be of negligible worth
A recent study provides some of the best evidence to date that birth order really doesn't have an effect on intelligence. The findings contradict many studies over the years that had reported that older children are generally smarter than their younger siblings. This new study, based on a large, nationwide sample, suggests a critical flaw in that previous research, said Aaron Wichman, lead author of the new study and a teaching fellow in psychology at Ohio State University.
Most previous studies compared children from different families, so what they were finding were differences between large and small families, not differences between siblings, according to Wichman. "Third- and fourth-born children all come from larger families, and larger families have disadvantages that will impact children's intelligence," he said. "In reality, if you look at these larger families, the fourth-born child is just as intelligent as the first-born. But they all don't do as well as children from a smaller family.".....
But then the researchers analyzed the data using a variable that could take into account environmental differences between families. That variable was the mother's age at the birth of her first child. "Mother's age encapsulates many variables that could negatively effect the child-rearing environment. The younger a mother was at the birth of her first child, the lower we would expect intelligence scores to be within a family," Wichman said. That's because younger mothers would tend to have less education, more children, lower income, and other factors that would negatively affect the intelligence of their children. When the researchers controlled for mother's age at first birth, the effect of birth order on intelligence was nearly eliminated.
The results, though, are clear, Wichman said. "Birth order may appear to be associated with intelligence, but that's only because larger families don't have the advantages of smaller families," he said. "When examined within families, there is no evidence of any significant association between birth order and intelligence. It's not your birth order that is important - family environment and genetic influences are the really important factors."
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