One study followed 1,300 children from their births in 1991 till the time they turned 15.
It found teens that attended child-care centres with higher-quality programs scored better on tests of cognitive and academic achievement than those who attended programs with lower-quality care.
The teens in higher quality daycare centres also had fewer behavioural problems such as rule breaking, arguing and hanging out with peers who get into trouble.
The finding was consistent among children from middle-class, low-income, two-parent as well as single-parent families.
"This evidence of long-term effects of early child-care quality is noteworthy because it occurred in a large economically and geographically diverse group of children," notes study author Deborah Lowe Vandell, a professor and chair of education at the University of California, Irvine.
However, teens that spent the most hours in child care during their first 4½ years reported more risk-taking behaviour and greater impulsivity than those who spent less time in child care.
The second study followed 1,300 children from birth to the fifth grade.
It found parents who visited their child's school and encouraged educational progress in the home had children with fewer behavioural and emotional problems.
The study shows those children had fewer aggressive and disruptive behaviours, and less anxiety and depression.
At the same time so-called pro-social skills such as co-operation and self-control were higher.
However, the high rate of parental involvement didn't seem to improve a child's academic achievement.
The second study was conducted at the University of Pittsburgh.
Both studies are published in the May/June issue of the journal Child Development.