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Birth Order Influences IQ?

Birth order is quite a buzzword while accounting for differences in personality, social behavior, the kind of profession you pursue to even the IQ levels of siblings. Birth order is defined as a person's rank by age among his or her siblings. It is often believed to have a profound and lasting effect on psychological development and believed to wield an influence on a person's IQ level.

In 1973 Lillian Belmont and Francis Marolla conducted a research encompassing nearly the entire population of 19 year old Dutch men comparing family size, birth order and IQ test scores. Slices of their report are annotated below:

  1. "Children from large families usually display a poor performance on IQ tests and on educational measures, even when social class is controlled."
  2. "Within each family size (i) firstborns always scored better on IQ tests than the later borns; and (ii) with few exceptions, there was a gradient of declining scores with rising birth order, so that firstborns scored better than secondborns, who in turn scored better than thirdborns, and so forth."
  3. "With increasing family size there was a general slump in IQ test scores within any particular birth order position." For e.g., a thirdborn born child from a 3-child family would be expected to score higher than a thirdborn child from a 4-child family. A thirdborn child from a 5-child family would be expected to score even lower, and so on.

The Confluence Model

Robert Zajonc, the late Stanford University psychologist was a potent proponent of the fact that birth order does impact a person's IQ. He has developed what has been termed the "Confluence Model" which shows that each successive sibling is born into a weaker intellectual environment and that intellectual performance increases with decreasing family size. When the gaps are short children who are born early perform better on IQ tests that do later children.

Social Order or Birth Order?

The findings, published in the journal "Science", come after more than a century of scientific debate on whether birth order influences intelligence. However, the key to higher IQ seems to be not being born first, but growing up as the senior child in the family. A new report collaborates that how siblings are raised, not their birth order, is what matters when it comes to brain power The study puts forward the claim that brain power and IQ score is dependent on the social rank in the family not birth order as such.

Dr. Petter Kristensen of the National Institute of Occupational Health in Oslo, Norway, and a colleague studied the birth order, IQ, and vital status of elder siblings of more than a quarter million 18- and 19-year-old male Norwegian military draftees. The researchers tried to eliminate the biological effects of birth order from the effects of social status within the family by comparing the IQs of young men who had experienced the early loss of an older sibling and those who did not.

"In this way," Kristensen elicited, "we were able to identify men who had first rank in social terms but second or third rank in biological terms and men who had second social rank in the family but were third born. For comparison, they included in the study men who were biologically born first, second, or third and had not witnessed the early loss of an older sibling.

The investigators found that children raised as the eldest showed slightly higher IQs than their younger siblings. Even if a child had lost an older sibling and was raised as the eldest, their IQ was higher by an average of 2.3 points than their younger siblings. This effect may also be observed in siblings who, although later born, have a sibling at least five years senior with no siblings in between. These children are considered to be "functional firstborns". This strongly implies, Kristensen reported, that the association between IQ and birth order is related to the social rank rather than the biological birth rank.

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