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POSTED 1 FEB 2001 If you're looking for smart children, grow 'em big. That's the boiled-down message from a new study in the United Kingdom that followed the intelligence of people born in 1946.

big baby behind new york skyline (AKA King Kong) with planes and copters flying all around it.The study found, unsurprisingly, that bigger babies become smarter children. But it was one of the first to find this relationship among babies with normal birth weights. Previously, most studies had focused on babies weighing less than 2.5 kilograms (5.5 pounds).

From these studies of low-birth weight babies, "It's well known that they are at risk of poor intellectual development," says Marcus Richards, a researcher at the UK's Medical Research Council who directed the new study. "We showed an association in a group representing normal birth weight, so whatever is driving the association [in tiny babies] must be acting in the normal range, too."

Bigger is brainier
The study grew from a farsighted decision by British researchers to hound 5,362 babies -- all born during one week in March, 1946 -- over the course of their lives. The scientists have tracked the cohort's physical and psychological health ever since. Due to a lack of data, the gestational age of these children -- how well matured they were at birth -- was unknown.

The specific intelligence tests were selected as appropriate to different ages:

Age 8: reading comprehension, word pronunciation, vocabulary, verbal reasoning

Age 11: verbal and non-verbal intelligence, arithmetic, word pronunciation, vocabulary

Age 15: verbal and non-verbal intelligence, reading comprehension, mathematics.

Age 26: reading comprehension

Age 43: verbal memory, timed letter search.

The new study considered only subjects in the "normal" range of birth weight -- 2.5 to 5.0 kilograms. To reduce extraneous factors that might affect intelligence, sex differences, birth order, father's social class, and mother's education and age were all statistically removed from the results.

The data showed a relatively straight-line benefit from higher birth weight. In short, bigger is better throughout the normal range of birth weight. This remained true even when low birth-weight babies were removed from the analysis.

Head starting young
The biggest benefit of colossal cubs appears early -- heavy babies were already smarter in the first tests, at age 8. And while that trend continued at ages 11, 15 and 26, the advantage apparently reflects the earlier disparity, Richards says. "It looks as if birth weight puts you on a trajectory by age 8, and you stay there until at least early adulthood. There's no sense that birth weight gives you an increasing advantage over age."

Not that you could do much about it anyway...

By age 43, in fact, the relationship between bulky birth weight and test scores breaks down. Bigger babies are no longer smarter adults.

Although this leveling could simply reflect the use of different tests at different ages, Richards thinks it's more likely that the small benefit of birth weight is eventually erased by the environment: "People may have caught up over time because there's an increasing number of things that influence cognitive function -- education, occupation, health."

The research adds to the evidence on the lasting benefits of good fetal development. Richards says David Barker, at the University of Southampton, found an association between low birth weight and a range of medical problems in adults, including diabetes and cardiovascular illness.

Big is beautiful
All other things being equal, the larger the better for intellectual development.Why? Epidemiology can never answer this, the ultimate question, but Richards says the present study, "suggests that something going on in early life to do with physical growth also influences the brain." Perhaps growth factors that produce large babies also "target areas of brain that are important for cognitive functions."

Richards cautions that the notion remains "the most interesting hypothesis," not a proven explanation. In other words, it's good cocktail conversation, but he wouldn't recommend buying stock in a company that promises to market growth factors to over-eager parents.

Since women are already encouraged to have big babes, the practical implications of the study may be limited. For his part, Richards says he's eager not to guilt-trip parents-to-be. "The last thing I'd want is for mothers with less than average birth weight babies to worry...Let's put it in context. There are numerous things that influence cognitive development in children, and some are far more powerful than birth weight." Parental involvement in education, he suggests, can easily compensate for the intelligence effect of birth weight. "I would not want anyone to feel alarmed by this."

However, it's also true that larger babies went further in school than small ones, suggesting that the test results are not merely theoretical.

There are many reasons to avoid small babies. Now it could be that enormous infants have an advantage over average ones. "Larger the better in terms of intellectual development, all other things being equal," Richards says.

-- David Tenenbaum





Birth Weight and Cognitive Function in the British 1946 Cohort Study: Marcus Richards et al, BMJ, 2001: 322, 199-203.




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