parental instinct?

If our thinking is totally material, based on “stuff” that is already “wired” in our bodies, then how do you explain the physiological and emotional response of adoptive parents? Let’s say parents who have genetic connections to their offspring have in place mechanisms that “imprint” on these children. Call it instinctual. I don’t know if there is research to this effect, but let’s just say there is.

Now add to the mixture adopting a child. If you know parents who have adopted (recently friends Matt and Susan, for instance), there is an immediate imprint that most often occurs. Where does this physically come from? An instantaneous rerouting of brain circuitry to override the genetic-based imprinting?

Allow me to make a disclaimer here. I am using the logical fallacy of “begging the question”, as I am assuming someone has already done studies on instinctual connections between parents and biological children, and this assumption has to be accurate for my question about adopted children to be correct. If there is indeed some physiological source for parental imprinting, then where is the source for parental imprinting on adopted children?