Adopted children as a natural experiment ?

Many applied micro-economists like natural experiments to deal with selection on unobservables and the use of data on adopted children (compared to non-adopted) seems a nice way to deal with with unobserved genetic effects. For example see Erik Plug's AER paper "Estimating the effect of mother's schooling using a sample of adoptees" (2004), 94(1), 358-368.
Such a model essentially relies on the assumption that the only difference between biological and adopted children is that the parents are genetically related to one and not the other.
But adopted children may differ systematically for other reasons and the behaviour of parents towards such children may also differ so its unclear to me that this is a good natural experiment. Parents do have biological children by accident for example whereas adoption takes a big investment of time and energy and also requires the consent of others. Consistent with my skepticism about the merits of this "natural experiment" (I think) is the following paper:
Differential parental investment in families with both adopted and genetic children Kyle Gibson
Abstract:
Stepchildren are abused, neglected and murdered at higher rates than those who live with two genetically related parents. Daly and Wilson used kin selection theory to explain this finding and labeled the phenomenon “discriminative parental solicitude.” I examined discriminative parental solicitude in American households composed of both genetic and unrelated adopted children. In these families, kin selection predicts parents should favor their genetic children over adoptees. Rather than looking at cases of abuse, neglect, homicide and other antisocial behavior, I focused on the positive investments parents made in their children as well as the outcomes of each child. The results show that parents invested more in adopted children than in genetically related ones, especially in educational and personal areas. At the same time, adoptees experienced more negative outcomes. They were more likely to have been arrested, to have been on public assistance and to require treatment for drug, alcohol or mental health issues. They also completed fewer years of schooling and were more likely to divorce. In adoptive families, it appears that “the squeaky wheel gets the grease.” Parents invest more in adoptees not because they favor them, but because they are more likely than genetic children to need the help. I conclude that discriminative parental solicitude differs in adoptive and step households because adoptive families generally result from prolonged parenting effort, not mating effort like stepfamilies.
Evolution and Human Behavior 30 (2009) 184–189