Another strength of this study is that it was a longitudinal study. This means that the adoptees were studied over a long period of time. Rutter tested the adpotees at different ages (4,6,11 and 15) and their physical,cognitive and social development was assessed. This suggests this study has a large quantity and variety of data. Therefore, this increases the reliability of the study and it can be seen as a strong study of attchment.