Are twins stupid?
A study from the BMJ says maybe:
At age 7, the mean IQ score of twins was 5.3 points lower (95% confidence interval 1.5 to 9.1) and at age 9, 6.0 points lower (1.7 to 10.2) than that of singletons in the same familyOf course twins tend to be born earlier and smaller than single births, which could account for some of the discrepancy. Indeed, correcting for these factors narrowed the gap to 2.5-4 IQ points with confidence intervals that overlapped zero and hence could have been due to chance. Of course, this only explains the difference, doesn't eliminate it.
Even 6 IQ points is not a huge number for an individual, and these are averages. Nonetheless, I'd think this study merits some consideration when we think about the effect of fertility treatments which have led to an increase in twins, triples etc.