What age were these people adopted at? If the basic personality of a child is strongly determined by anything that occurred before the adoption date, (not just genetics,) then you wouldn't expect subsequent parenting to have much effect.

Given that many developmental psychologists (John Bowlby 1969, Hazan and Shaver 1987, Grossman and Grossman 1991, Quinton Rutter and Liddle 1984... and so on) found evidence to support the ideas that, among other things, infant attachment styles predict later friendships, later happiness in love, and eventual parenting styles - it seems likely that, even if the children were adopted quite early on in life, adoption studies wouldn't adequately differentiate between degrees of genetic and environmental influence.