One limitation is that sources of evidence for maternal deprivation are flawed. The 44 theives study is flawed due to open bias as Bowlby himself assessed both deprivation and psychopathy, knowing what he hoped to find. Also, Goldfarb's study of wartime orphans is flawed as he used traumatised participants who lacked good aftercare. This introduced confounding variables. This means that Bowlby originally had no solid evidence on which to base his theory of maternal deprivation.