Our genes do influence the development of brain structures that strongly influence general happiness level, sociability, and a variety of other traits. Under normal circumstances, the wiring of those neural structures tends to be reinforced. For example, friends and family members might observe a loved one who is not by nature sociable and say "you are an introvert" or "you are shy" which is a reinforcing stimuli. When that person has heard this enough times, whenever they behave in an unsocial manner, they will tend to explain it by thinking "I'm shy/an introvert" thus perpetuating the neural structure. If instead, sociability is treated as a skill, and correct behavior is reinforced while incorrect behavior is punished, over time those neural structures will be re-wired.
This pattern is observed all the time for tastes. We have genetically predetermined preferences, but overwhelmingly what we end up liking is the result of social conditioning.
That doesn't really seem to address the issue of adopted children though. If I understand you correctly, the social environment (and conditioning) of the children should override the biological aspects of their personality. Or at the very least it should have a very significant effect.
It seems to me that certain aspects of a person are very malleable, taste being one of them. But many other things apparently are not (IQ, or various other measures of 'intelligence', if I recall correctly).
From my experience in the social sciences, we tend to underestimate the role of genetics more often than not.
It does address the issue of adopted children - we as a culture have a fixed mindset for things like introversion and intelligence, and genetics do strongly influence our default level of both of those. Thus, adoptive parents observe the child and say "you're an introvert" or "you're not clever" and the default pattern is reinforced.
On the other hand, when a child displays an initial dislike for like Taylor Swift, her friends might say "you're crazy, Taylor Swift is awesome."
It's all just neural networks being reinforced or inhibited, the primary contribution of genetics is the default state of the network. In the case "fixed" personality characteristics, our culture is to reinforce the default networks, while for things we view as mutable, we have no qualms about training away default behavior.
Why should "introversion" be changed or punished, necessarily? Developing intellect is something to be encouraged, but I don't understand the anti-introversion bias in your post.
And I suspect that the genetics does also set a limit as to how far one can bend, too. This too, will vary between persons.
This pattern is observed all the time for tastes. We have genetically predetermined preferences, but overwhelmingly what we end up liking is the result of social conditioning.