(distinguishing related individuals who share genetic ancestry, for example twins, from individuals who share household environments, for example adoptive siblings) by its nature as a sample of children all born in short time window. But the data are interesting nonetheless.
http://www.jaacap.com/article/S0890-8567%2809%2963427-4/abst...
http://onlinelibrary.wiley.com/doi/10.1111/1467-8624.00398/a...
(distinguishing related individuals who share genetic ancestry, for example twins, from individuals who share household environments, for example adoptive siblings) by its nature as a sample of children all born in short time window. But the data are interesting nonetheless.