While I don't discount the genetic factor, I do not consider it the primary reason why Christianity is failing in the West. I cannot speak about Islam, but only Christianity. I see the failure of Christianity as a result of increased secularization and the values of universal egalitarianism on the rise. Christianity and it's success depends on a level of commitment like that which the early Roman Christians exhibited. A willingness not only to die a martyr, but to exclude from one's own group those who would destroy the integrity of the group.
One of the only groups of Christians that is actually growing are the Amish. I would not count them as the most gifted people intellectually. They are simple and yet the way of life they live is one wholly dedicated to religion. God is their first loyalty, followed by their immediate community and then everyone else. They have strict rules in place to manage their society and those who break the rules are shunned. Modern Churches have problems even excommunicating known and open heretics. In effect, Christians have become too attached to being part of the wider society. They are less particular about themselves as Christians and more likely to think of themselves as Americans, New Zealanders, Australians or whatever. Religion is not their first loyalty.
As for the idea that religion cannot help those with psychopathic tendencies, if that's what I understood you to be describing, I don't agree. Psychopaths might be less willing to convert and might not feel the same as the rest of us but religion can make a difference in their lives if they are convinced of it. I think of David Wood, a serious and committed Christian apologist who once tried to murder his own father. Even if the psychopath doesn't have the same emotional reaction, a commitment intellectually to the rules and willingness to follow them can lead to lasting benefits.