New research published today by researchers at the Institute for Fiscal Studies has found "little or no evidence that marriage itself has any effect on children's social or cognitive development". It is true that, on average, children born to married parents achieve better 'cognitive and social outcomes' than children born into other family forms, including cohabiting unions, but the research indicates that this is simply due to the fact that more affluent and better educated couples were more likely to get married. Accordingly, the IFS concludes that: "Policies aimed at encouraging parents to get married before they bear children thus require a rationale other than one based on the impact of marriage on child development".
It will not make very good reading for David Cameron (although I suspect he currently has other things to worry about) or, indeed, for Iain Duncan Smith and the Centre for Social Justice, which has long campaigned for policies aimed at encouraging marriage.
You can read the research here, a press release from the IFS here and a report about it in The Telegraph, here. There is also an article about it in The Guardian, here.