Overview: When twins are born at markedly different weights, the smaller of the set is three times more likely to develop autism than his or her sibling, report researchers in the December online issue of Psychological Medicine.

Background: Earlier this year, a study using single births found a correlation between low birth weight and increased autism risk. Northwestern University researchers have now studied both identical and fraternal twins to further examine the autism and low birth weight correlation.

What's new: In the study, the correlation between lower birth weight and an increased risk of developing autism held true whether the researchers were comparing identical or fraternal twins.

Why it's important: The researchers' findings suggest that environmental factors, rather than genetic, are the basis for the birth weight-associated risk difference.

What Evidence Supports a Role for the Environment in Autism?

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