IQ Scores: Different for Twins?

If So, Early Birth or Low Birth Weight Could Be Why, Study Shows

Medically Reviewed by Louise Chang, MD on November 17, 2005
From the WebMD Archives

Nov. 17, 2005 -- In a study from the U.K., twins scored slightly lower on IQ tests than their single-birth siblings.

The small gap faded when early births and low birth weight were taken into account. As expected, the twins were often born earlier and smaller than single-birth babies.

But the researchers aren't totally satisfied with the findings. They didn't get all the information they wanted, and the study's scope was limited. They call for more work on the topic.

The researchers included Georgina Ronalds, MSc, of the London School of Hygiene and Tropical Medicine. The study appears in BMJ Online First.

About the Study

Data came from a citywide study done in Aberdeen, Scotland, in the 1950s. Children in Aberdeen took IQ tests administered at school when they were 7 and 9 years old.

At age 7, single-birth children scored about five points higher than their siblings who were twins. At age 9, the gap had reached six points.

Twins' early births and lower birth weights may have been important, write the researchers.

But there's more to the story than that.

Pros and Cons

All comparisons were done within families. So twins from Family A weren't compared to single-birth kids from Family B.

That was done for consistency's sake. Lots of factors -- like family income and parental education -- were taken into account.

However, about a fifth of the children that the researchers wanted to study were left out. There was missing background information such as gestational age at delivery, which would demonstrate premature birth.

More current data, with tests given by IQ experts, would have been a plus, write Ronalds and colleagues.

It may not add up to a final verdict, but a possible hint about the benefits of a long, healthy pregnancy for any baby, twin or not.