Many types of genes influence intelligence

LOS ANGELES -- Scientists who hunt for "intelligence genes" used to think there were fewer than half a dozen of them.

In recent years, they determined there may be at least 1,000, each with just a tiny effect on the differences in people's IQ. A study released yesterday found new evidence that many genes play a role in intelligence, but the specific genes involved couldn't be pinpointed.

"It's been kind of a shock to the system that it hasn't worked," said psychologist Eric Turkheimer at the University of Virginia, who had no role in the study. "We can't find the effects of any individual genes that are large enough to seem worth worrying about."

Previous work involving twins and adopted children has found that genes have a significant influence on differences in IQ scores, producing about half the difference between adults in general. The influence of genes on IQ appears to grow from childhood to adulthood.

Scientists have come to realize that, as with height, differences in intelligence come not from a few genes, but rather the overall effect of many genes, each with only tiny influence. That makes them hard to tease out.

The new DNA study, reported online in the journal Molecular Psychiatry, came to similar conclusions. Many genes work together to shape intelligence much like the different instruments of an orchestra that play in sync. Unless a soloist is playing, it's often difficult to decipher the contributions of individual instruments.

As important as genes are in determining intelligence, they don't act alone and the role of one's upbringing and experiences cannot be ignored.

Our memory, reasoning skills and thinking abilities tend to decline as we age, some faster than others. Understanding the genetics of intelligence may someday help researchers gain a better handle on mind-robbing diseases such as Alzheimer's.

The new work was done by I.J. Deary of the University of Edinburgh, Scotland, and colleagues in several countries.

John Olsen, of Orange, Calif., who was adopted at birth, attributes his brainpower to his genes. As a kid, he always wondered where his inquisitiveness came from. School bored him and there were no lively debates at the dinner table growing up.

"I was a bit of a challenge," he recalled. "I was very curious and . . . always asked, 'Why?' " In a genius test in his late 20s he scored high enough to get accepted into Mensa, the high IQ group. Several years ago he was reunited with his biological mom.

Olsen discovered his mother had the same curiosity and liked to ask probing questions. He also learned his maternal grandmother was fond of one-line comebacks and "was wickedly smart till the day she died."