REUTERS - Mothers are the primary or sole source of income for a growing number of US families in a dramatic shift in American homes and the workplace from just a few decades ago, the Pew Research Centre said in a study released on Wednesday.

Based on data from the US Census Bureau, the study said a record 40 per cent of all US households with children relied on mothers as their main or only source of income. That marked a sharp increase from 11 per cent of families in 1960.

The so-called "breadwinner moms" are split into two camps, with married mothers who have a higher income than their husbands accounting for 37 per cent and 63 per cent consisting of single mothers, the survey said.

It said married women who outearn their husbands tended to be slightly older, disproportionately white and college educated and much better paid than single mothers, who were younger, more likely to be black or Hispanic and less likely to have a college degree.

Single mothers now make up one-quarter of all US households with children and women make up almost half of the US labour force.

According to the Pew study, the employment rate of married mothers with children has jumped from 37 per cent in 1968 to 65 per cent in 2011.