Still, size matters, in brain research at least as much as in brain function.

“I like this work a lot, because these guys finally did what needed to be done to take a real stab at merging imaging and genomics,” said Dr. Matthew W. State, a professor of psychiatry at Yale, who was not one of the collaborators.

Brain imaging studies are expensive and, as a result, far too small to reliably tease out the effects of common gene variations. These effects tend to be tiny, for one thing, and difficult to distinguish from the background “noise” of other influences. And brain imaging is notoriously noisy: not only does overall brain size vary from person to person, for instance, but so do the sizes of specialized brain regions like the hippocampus, which is critical for memory formation.

To solve the numbers problem, Dr. Thompson and three geneticists — Nick Martin and Margaret Wright, both of the Queensland Institute of Medical Research in Australia, and Barbara Franke of the Radboud University Nijmegen Medical Center in the Netherlands — persuaded research centers around the world to pool their resources and create one large database. It included genetic and extensive brain imaging results from about 21,000 people. The team then analyzed the collective data to see whether any genes were linked to brain structure. As the study was being completed, the Thompson group learned that another consortium, led by Boston University researchers, was doing a similar analysis using its own large group.

The two teams’ findings did not completely line up. One found size-related genes that the other did not. But they agreed on two findings: one gene that correlated strongly with overall brain size, and another that correlated with the rate at which the hippocampus atrophies, or shrinks, with age.

People who carried one variant of the overall-size gene had brains that were about 1 percent larger than those of people who carried another variant. The two variants are equally distributed — about half of people have one and half have the other.