books.google.co.ukhttp://books.google.co.uk/books/about/Free_Play_in_Early_Childhood.html?id=yJumYgTUDkgC&utm_source=gb-gplus-shareFree Play in Early Childhood
Page 89 - Cole, DA, & Carpentieri, S. (1990). Social status and the comorbidity of child depression and conduct disorder. Journal of Consulting and Clinical Psychology, 58, 748-757.