A study by Baydar & Brooks-Gunn (1991) examined the relationship between the time that mothers resumed work and the children’s Cognitive Development. They found that children of mothers who resumed work before they were a year old had poorer intellectual abilities and were more likely to have behavioural problems than children of mothers who started work later. This study supports Bowlby’s maternal deprivation hypothesis because separation from the mother before a secure attachment has formed may have meant that the children experienced distress and the behaviour problems may have arisen due to this distress. Poorer intellectual abilities are also predicted by Bowlby's maternal deprivation hypothesis, as a lack of a secure base means that children are less likely to explore their environments.
Another study by Belsky & Rovine’s (1988) study found that children who received day care for at least 4 months before their first birthday for more than 20 hours per week were more likely to develop insecure attachments. This also supports the maternal deprivation hypothesis because frequent, prolonged separations are expected to interfere with the development of an attachment.
However, it is possible that the findings of Baydar & Brooks-Gunn and Belsky & Rovine are misleading, as there is an important confounding variable in both studies: parents who return to work before the infant is 12 months old are more likely to come from lower socioeconomic groups and return to work because they cannot afford to stay at home. This means that these studies may be subject to many other confounding variables, such as parental stress levels, parents' lack of social support and educational level. Parent's educational level may have an impact because less educated parents may be less informed and, therefore choose lower quality childcare than more educated parents.
In April 2001, Jay Belsky hit the headlines in several countries with results from the Early Child Care Report, funded by the National Institute of Child Health and Human Development (unpublished at the time), which showed that children who received day care were more aggressive than those who did not. Moreover, the more day care they received, the more aggressive the children were. 17% of children receiving day care were aggressive, compared with 6% who had not received day care.
Nevertheless, the study also showed that day care children performed better in tests of cognitive ability and socioemotional development. Belsky’s interpretation of the data, however, has been strongly criticised by other developmental psychologists. His colleagues, Susan Campbell and Celia Brownell, at the National Institute of Child Health and Human Development (NICHD), for example, have argued that the 17% aggression rate is within the normal range. Moreover, the research was a correlational study, so it is tenuous to claim that daycare caused the increase in aggressiveness. It is possible that parents of difficult children are more likely to send them to daycare to give themselves a break.
Another criticism of Belsky's interpretation of the NICHD study, according to Campbell and Brownell, is that “aggression”, in terms of the study, encompassed a whole range of behaviours that could be construed in a different way: for example, seeking adult attention, clowning around, and talking a lot. Moreover, the average rate of aggressive children in the US (where the study was conducted) is 17% — exactly what Belsky found in his day care sample. It could be argued that children in day care are simply developing earlier than those not receiving day care.
