Detrimental effects [on mothers of six-month olds] of working outside the home are attenuated by the time the child is 4.5 years old.
In Early Maternal Employment and Family Wellbeing (NBER Working Paper No. 17212), authors Pinka Chatterji, Sara Markowitz, and Jeanne Brooks-Gunn study the effects of mothers' work outside the home on maternal depression, self-reported health status, parenting stress, and parenting quality. They find that among mothers of 6-month old babies, working mothers report higher stress and more symptoms of depression than non-working mothers. Working mothers also report less good health than their non-working peers.
On average, an increase of ten hours of work -- that is, a 40 percent increase in the average work hours for the employed mothers -- is associated with an increase of 6 to 9 percent in a depression score used by the authors. Hours of work do not appear to have any effect on the quality of parenting at six months, though; in this study, "parenting quality" is measured by ratings assigned by "trained observers" watching videotape of each mother interacting with her child in five semi-structured play situations at home and in laboratories.
The authors also examine similar measures of maternal outcomes four years later. It appears that any detrimental effects on mothers of working outside the home are attenuated by the time the child reaches 4.5. The authors conclude that "if anything, more maternal work reduces parenting stress during the first 4.5 years." They do not find any evidence of adverse effects of hours of work on maternal health among mothers with 4.5 year olds.
The data for this paper come from the Study of Early Child Care. It enrolled 1,364 healthy infants in 1991 and followed those children and their families from birth until age 15, interviewing them in person or by telephone every three months. When the children were 6 months old, their mothers were asked to complete a depression screening survey and were queried about their current levels of parenting stress, and their overall health. To measure parenting stress, the authors use the Abidin Parenting Stress Index and the Parent Role Quality Scale. To measure depressive symptoms, they use the Center for Epidemiologic Studies Depression Scale. The authors examine the scores on these scales relative to the hours that mothers reported working in an interview that was conducted when their child was 3 months old.