The Institute for Fiscal Studies have produced a report claiming that marriage does not benefit children's cognitive and social development. However, this does not provide conclusive evidence for the case against marriage as there are a plethora of studies which claim that the opposite is true.
For example, one study has found that unmarried couples are four times more likely to descend into poverty,
Click here for The Institute for Fiscal Studies report.
The Centre has a section on marriage and well-being here