BAKERSFIELD, Calif. -- More than 40,000 American children are born each year using assisted reproductive technologies such as in vitro fertilization. But what becomes of these kids as they grow up?

Researchers from the East Virginia Medical School surveyed 173 young adults to find out. They found kids conceived through IVF were overall quite similar to other people in their early 20s -- with typical rates of obesity, birth defects and cancer.

In some ways, they were even healthier -- just 14 percent were current smokers compared to 39 percent of young people the same age in the general population.

But researchers were troubled by an unexpectedly high rate of psychological problems among those conceived via IVF.

Nearly one-third had been diagnosed with a mental disorder, most commonly ADHD or depression. The depression rate was only slightly higher than normal, but the odds of ADHD were more than 5 times greater.

Researchers speculated that fertility issues may cause greater stress on parents, which may result in more behavioral problems among the children.