Nature vs Nurture has been a debate in child development for decades. When I started my adoption journey, I believed nurture conquers all. I was wrong. Among the factors it doesn't conquer is prenatal alcohol exposure, mental illness that is present in both biological parents, genetic disorders, and congenital diseases.
When you chose to conceive a child, you have some control over these factors. You can chose not to drink. You probably know if you or the child's other parent battles a mental illness. Some genetic disorders can be tested for, some are spontaneous mutations. Congenital diseases don't give you a lot of ways to prevent them, but you do what you can do, like taking vitamin C to help prevent spina bifida.
When you adopt an infant, all you can do is ask questions. If the birth mother doesn't want you to know there isn't much you can do. The rate of mental illnesses in children adopted at infancy is significantly higher than for biological children. The speculation is that people who chose adoption for their newborn are more likely to live with a mental illness and self medicate with alcohol and other drugs. Researchers are trying to isolate various factors, but given the nature of adoption historically it is proving difficult. I know that GB's birthmother still denies using alcohol during the pregnancy, even with scores of people who say they saw her and GB's facial features. She denies she is mentally ill even though she is receiving SSI for being Bipolar.
When you adopt an older child, you get to throw in PTSD and attachment issues, and if you hit the jackpot, even RAD.
Although I am a firm proponent of scientific, evidence based interventions, I also believe each child, no matter how they enter your family, is a gift from God and we are expected to help them fulfill the purpose God has for them. Contradictory? Yes, but still what I believe.