Marriage confers "little if any benefit" in terms of a child's development, according to new research, challenging the rationale behind the prime minister's desire to offer tax breaks to couples who tie the knot.

New research by the Institute for Fiscal Studies (IFS) has found "little or no evidence" that marriage itself has any effect on children's "social or cognitive" development. Before the election the Tories had made the breakdown of the traditional nuclear family a key plank of their "broken Britain" analysis.

David Willetts, the Conservative thinker on families and now higher education minister, argued that marriage in Britain was in danger of becoming an exclusive middle-class institution – and action was needed bolster it. The idea is still floated by key Tories, such as work and pensions secretary Iain Duncan Smith, but is opposed by Lib Dems.

The work by the IFS accepts that those who marry tend to be relatively better educated and relatively better off. But the institute points out: "differences in outcomes between children whose parents are married and those who cohabit may simply reflect these differences in other characteristics rather than be caused by marriage."

By examining data from the Millennium Cohort Study, a sample of children born in the UK in the early 2000s, the institute shows that children born out of wedlock are behind in cognitive development at three, five and seven-years-old but this is because "cohabiting parents tend to have lower educational qualifications than married parents". The same pattern is observed with "socio-emotional" development.

The thinktank said it had also repeated its work using another dataset to account for the idea that "getting married could itself lead to changes in some of the things we want to control for, like relationship quality, income and education". The results were the same.

Ellen Greaves, research economist at the IFS, and one of the authors of the report, said: "It is true that children born to married couples are on average more cognitively and emotionally successful than children born to cohabiting couples. But careful analysis shows that this largely reflects the differences between the types of people who decide to get married and those who don't."