Young children looked after by their mothers develop better than those cared for in nurseries, or by relatives or childminders, research suggests.
Children developed less well at nursery schools
About 1,200 children were monitored from birth to the age of three.
The social and emotional development of babies who were cared for by someone other than the mother was "definitely less good", the report's authors said.
The study, by Oxford University and the University of London, was one of the biggest so far on UK childcare.
Mothers were seen as the best carers, followed by nannies and childminders, then grandparents, and nursery care was the worst, the study said.
Study author Penelope Leach said the results were not a call for all mothers to stay at home and give up work.
Instead, they highlighted a demand for "developmentally appropriate high-quality childcare".
Half of the children observed by Dr Leach and co-authors Kathy Sylva and Alan Stein were cared for full-time by their mothers, a third had mothers who returned to work after seven months and 8% had a mother who went back to work after three months.
The study began in 1998 and involved children from London and Oxford.
The babies were seen at three, 10, and 18 months, and again at three years old and just over four.
Children not cared for mainly by their mothers also tended to show higher levels of aggression and became more withdrawn, compliant or sad, the study suggested.
"The study does not mean that every child in a large nursery will become a monster," Dr Leach said.
"Nevertheless, it shows a small but significant difference in a large group of children."
She also said that a "staggering" number of mothers did not follow up the references of their child carers or organised care at the last minute.
"Mothers also often wanted their own mother as the carer because they say `she's family, she loves the baby'," she said.
"But love doesn't necessarily produce the best childcare. That takes planning and thinking about the child and his or her activities."
However, not all mothers were the best at providing care.
Those suffering depression or with priorities other than motherhood would be better leaving their child in another's care, Dr Leach said.