A 2011 study analyzed the development of almost 19,000 British children and found that, compared to children born to married parents, children born to cohabiting parents had deficits in cognitive and socio-emotional development at ages 3, 5 and 7. However, these deficits were largely accounted for by their parents’ generally lower educational qualifications and income levels rather than any specific factors arising from differences in marital status.
This 2011 study also looked at data tracking the entire lives of people born as long ago as 1970. This data was paired with developmental data taken from these participants' children. This analysis found that, when accounting for the parents’ backgrounds, there was no difference in children’s cognitive or socio-emotional development regardless of their parents’ marital status.