Preschoolers whose mothers worked more than 30 hours a week during their infancy tend to have poorer outcomes than those whose mothers worked part-time or not at all until the children were nine months or older, according to new research from Columbia University. The former group had poorer cognitive and verbal development by age three, even when other important factors like quality of child care, the home environment and maternal sensitivity to the child were held constant.
The authors argue that the findings should spur public debate not over whether mothers should work, but on the business and public policies that work best for families, so mothers can work fewer hours when their children are very young.
http://www.columbia.edu/cu/news/02/07/working_mothers.html