Look at the effect of childcare on American children
It was a longitudinal study using observations, interviews and surveys. It studied 1200 children from birth until when they started school.
Children that spent continuous time in daycare, were more likely to have behavoural problems later in life, so the length of time that is spent in daycare is important.
Nursery care improved cognitive and language development but increased the possibility of behavioural problems (e.g. agression).