Although the effect of head size on IQ is minimal, it does exist, says Jeremy Genovese, who conducted the new research and is an associate professor of human development and educational psychology at Cleveland State University.

"The correlations between head size and IQ are quite modest, and you cannot determine someone’s intelligence with a tape measure," he told Discovery News. "However, the correlation is real and might have some clinical significance, such as predicting susceptibility to dementia."

Genovese explained that "larger bodies do require larger brains to support larger nervous systems," but he added that the notable difference in body size between men and women appears to have "no relationship to intelligence."

For the study, which has been accepted for publication in the journal Personality and Individual Differences, Genovese obtained copies of the 1939 inmate data, which was collected by Harvard anthropologist Earnest Albert Hooten. Hooten gathered anthropological and sociological records on roughly 12 percent of American prison inmates.

Included in Hooten’s study were head size measurements of 676 male inmates at the Concord Reformatory in Massachusetts, since IQ tests had already been performed on the same prisoners. Hooten documented head circumference, head length, head height and even head shape.

Genovese used statistical computer software to find patterns hidden from Hooten, considering the more limited methods of his day.

Other scientists have linked head shape to higher IQs, arguing that a rounder head is associated with greater intelligence, but Genovese could find no such link. He was, however, able to confirm what Hooten suggested, that there is a connection between head size and intelligence. But teasing out that connection is anything but simple.

Since size is one indicator of overall physical fitness, Genovese speculated that a fitness factor could be influencing the results. A fit person, for example, could be more likely to have both a larger head and smarter brain, with fitness (rather than head size) being the direct cause of intelligence.

It’s hard to say for sure, said Genovese.

"It is quite possible that greater body size, head size and IQs are, in this sample, proxies for some other variables, such as nutrition or socio-economic status," he said.

Over the years, other researchers have looked at the possible link between head size and intelligence, and the results have been mixed. Michelle Luciano, a researcher in the Genetic Epidemiology Laboratory at the Queensland Institute of Medical Research, recently tested 4,395 teenagers for that link but could find no such correlation.

On the other hand, Laura Flashman of the Dartmouth Medical School did identify "a significant positive relationship" between IQ levels and measurements of the frontal and temporal regions of the brain.

Colin Groves, professor of biological anthropology at the Australian National University, contends that human brains have actually become smaller since the late Pleistocence, around 11,000 years ago. Groves agreed that head size may be linked to intelligence, but only between species and not within a certain species.

Since Neanderthals had big heads, it could then be that their smarts have been underestimated.

Genovese said, "I rather think that the Neanderthals get a bad rap."

Via: Discovery Channel