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Does Birth Order Affect Intelligence?

PS2007's picture

Does Birth Order Affect Intelligence?

                There have been many hypotheses about how birth order affects children’s personalities. Many generalizations have been made about this subject, including the ideas that older children respect authority and younger children are rebels (1). Most of these theories have not been supported by scientific research, but this summer I read an article in The New York Times that stated that new research had found that older siblings tend to develop a higher IQ than later born siblings. Being an older sibling myself, I was curious to find out if this was true.

                The article in The New York Times focused on a recent study completed by Norwegian Epidemiologists that claimed that on average, older children have an IQ that is three points higher than younger children. This may not seem like a lot, but over the course of a lifetime those points can add up to equal higher grades, better jobs, and a bigger paycheck. The researchers do not blame genetics, but family dynamics for this difference in IQ. This is because the IQ trend stayed the same in families where the oldest child had died and researchers tested the second oldest child (who was now the oldest). Critics of the study cited the fact that researchers did not take into account differences in upbringing among families. In addition, there is no information regarding the impact of the death of a sibling or child in a family and how it may affect or disrupt the kinds of environmental factors that contribute to IQ development. Also, the difference in IQ is the average of the siblings tested, meaning the sometimes the differences were a lot greater and sometimes they may not have existed at all (2).

                In researching more articles to see how The New York Times article compared to other studies, I was surprised to learn that whether birth order affects intelligence is a very heated and controversial subject. There does not seem to be substantial evidence to support either side, but scientists who have formed views on the subject defend their side passionately. Researchers Wichman, Rodgers and MacCallum who have spent time studying this question say that birth order has no affect on intelligence levels, and that the differences we see in IQ have more to do with the fact that much of the data collected has been cross sectional and between families instead of intra-familial. They say that the differences that these studies show in IQ are really differences in IQ between large and small families. Small families have more advantages and because of them score higher on IQ tests. Their research shows that if you test all children in a small family and all children in a large family within families the IQ would be the same but the smaller family would have a higher IQ. They concluded their study by saying that the factors that really influence IQ are genetics and family environment (3).

However, two other researchers of this subject, Zajonc and Sulloway, criticize the study performed by Wichman, Rodgers and MacCallum by stating that their tests were unfocused and they ignored statistically significant data (4). Zajonc originated the confluence model, which explains that older children score higher on these IQ tests because they have mostly adult influences around them in their early years (1).

After reading about these studies and researching about this idea, I still do not really know whether birth order affects intelligence, which means I agree with most of the scientific community on this issue. I think it would be interesting to research the importance of genetics when it comes to intelligence. I think if I understood the genetics of IQ this would help me decide whether I believed IQ varied between siblings or between families. One thing I gained from researching this subject was learning how important it is to understand the methods of the study, because a way a study is conducted and analyzed can radically change the results. I will be interested to see the studies about this subject in the future, although I wonder if we will ever be able to find conclusive evidence to support either side of this argument.

Works Cited



3) Wichman, A. L., Rodgers, J. L., & MacCallum, R. C. (2007, September). Birth Order Has No Effect on Intelligence: A Reply and Extension of Previous Findings. Personality and Social Psychology Bulletin, 33(9), 1195-1200.

4) Zajonc, R. B., & Sulloway, F. J. (2007, September). The Confluence Model: Birth Order as a Within-Family or Between-Family Dynamic? Personality and Social Psychology Bulletin, 33(9), 1187-1194.