Although much of the research has been conducted on populations in other countries, Berger et al. found that children in the United States whose mothers return to work within the first 3 months after giving birth are less likely to be breastfed, have all of their immunizations up to date (by 18 months), and receive all of their regular medical checkups; they are also more likely to exhibit behavioral problems by four years of age. Chatterji and Markowitz also found an association between longer lengths of maternity leave and lesser incidence of depression among mothers.