Does the month of your birth affect IQ?

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For over 70 years, research has shown a relationship between season of birth and intelligence or educational achievement. Although some research results vary, the bulk of the data suggests that learning skills and birth season are linked. However, the jury is out as to why month of birth might be related to IQ. Debbie Lawlor and her colleagues examined some data to confirm the link between birth season and intelligence and to find out why such a relationship might exist.

Two theories have been proposed to explain the relationship. One suggests that environmental factors during and after pregnancy affect brain or neurological development. Temperature in utero, for example, might have an impact, along with seasonal variations in maternal nutrition. Perinatal infections might also affect development. Some researchers have found a link between birth weight and later intelligence, and it's certainly true that some intrauterine factors can affect the weight of an infant at birth.

The second possible explanation for the seasonal variations in IQ is linked to school policies around age at school entry. Schools that have a single cut-off date for entry into Kindergarten will have a 12-month range in the age of students. Older kids might be able to get more out of the curriculum than the younger, less mature students; this may be related to cognitive development as well as an increased ability to regulate behaviour. Lawlor wrote about other school systems in which children begin attending in the semester in which they turned 5, but leave primary school according to their year of birth; in these school boards, students born in the winter spend less time in school than their summer-born classmates. Since educational background can affect performance on measures of intelligence and school achievement, it's possible that age at school entry or age relative to classmates might explain the relationship between birth season and IQ.

Lawlor and her colleagues examined information gathered from over 12,000 students. At the time of the data collection, schools in the area followed the "three times per year entry policy," so students born between April and August would have had the most school experience and those born between January and March the least. Lawlor found small seasonal variations in abilities, but once age at school entry and age relative to classmates were taken into account, the differences disappeared. So at least for young students, seasonal intelligence and achievement patterns appear to be related to age of school entry rather than other environmental factors.

The study's authors were surprised, however, that the small seasonal differences were not in the direction that they expected. Students who entered school later in the year, and therefore spent less time in primary school, scored HIGHER than those who started school earlier. The researchers suspected that individual attention provided by parents may have been more effective than the extra semester of group instruction provided to the kids who started school earlier.

What can you take away from these findings? First, it might be reassuring to parents that, at least when skills are measured at a young age, month of birth doesn't appear to have a major impact on intelligence or academic performance; seasonal differences seem to be related more strongly to instructional experience (individual attention provided, age of school entry) than to the timing of one's birth.

You can read Lawlor's study here.

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