I work with a neonatologist who specializes in the perinatal follow-up of premature infants (at least until the age of three). He has accumulated years of data in our province. The very first question he asks when learning a baby's history is "Are the parents married?" Factoring out all other variables, this is the most consistent one in the cognitive and mental development (as measured via the Bayley's Developmental Assessment) of the prems in our area. Says something about the importance of a stable social environment in these situations.
Maybe to some extent, but I think the primary reason for the effect may be that low-IQ parents are more likely to have a child out wedlock -- in a study by Murray and Herrnstein,
http://en.wikipedia.org/wiki/The_Bell_Curve , 32% of the mothers with IQ < 75 were unwed, compared to only 2% of the mothers with IQ > 125 -- and that IQ is about 75% heritable in the U.S. (and in Canada, I'd guess). Therefore children born out of wedlock tend to be less intelligent.
