As a science fiction fan I occasionally read a science fiction novel written by a non science fiction writer. They usually are not very good. The author usually does not understand the genre and it shows. They usually try to downplay the fantastic elements or the scientific extrapolations as if they are embarrassed about it. The story often turns into a soap opera with the scientific extrapolations appended as if the author was forced to put them in against his will.
The Third Twin is an exception. I suspect that the reason that it was good is that Ken Follett did not even know he was writing science fiction and so he did not feel out of his element. He seems to have thought that he was writing a contemporary thriller that drew on the headlines that were current in the mid 1990s when he was writing it. He was not incorrect either. However, he includes a bunch of human clones as essential characters and this is science fiction whether he was aware of it or not. The book is a bit misnamed because there are more than three of the clones; there are eight of them. Only two of them become major characters and three of them are minor characters with the others discovered to exist and one of them has died before the story starts.
The question at hand is how much of personality is inherited and and how much is due to the environment. While twin studies have been done that hint at some answers there is little that is conclusive because identical twins who have been raised apart are not very common and no one has yet produced eight human clones. Everything tends to point to a combination of both nature and nurture as shapers of personality though. It must be remembered that genes must interact with environment to do anything so one should be very careful about downplaying external causes of personality traits.
In this story one clone turns out to be a vicious rapist and another one turns out to be a sensitive nice guy. The other clones, while exhibiting various degrees of successfulness and viciousness, are all, at least, potentially asocial. The one thing I really liked in this novel was when the nice guy clone comes to an epiphany. While wondering if his upbringing made him the man he is or his genes he realizes that the main factor were his own choices. That is something that is almost always missed in the nature-nurture debates. Everyone is saddled with his or her set of genes and environment and have not much choice about that. However, everyone can make one's own decisions too and that has a lot to do with how a person turns out.