Kids share more than candy and germs when they're growing up. Siblings of kids with emotional or behavioral problems tend to have those same types of problems themselves.
Conduct problems and emotional disorders are especially likely to run in families, according to Canadian psychiatrist Peter Szatmari, M.D. Kids with attention deficit disorder need not necessarily have siblings with similar problems, but it's very likely that these kids and their sibs will have conduct problems or emotional disorders as well.
The findings suggest that nature and nurture are in cahoots once again. Siblings may share a general genetic vulnerability for psychiatric problems, while environmental factors determine which specific problems develop.
The double whammy of environmental and genetic similarities within families helps explain why conduct problems and emotional disorders cluster with such high frequency.
Why not attention deficit disorders? Szatmari, of Chedoke-McMaster Hospitals in Ontario, suspects that hyperactivity is purely genetic--and not doubly triggered by the interplay of genes and family environment. Kids may also "catch" problem behaviors from each other.