Perhaps this book should be called "The Faith Gene" instead of "The God Gene." Geneticist Dean Hamer himself admits that "The God Gene is in fact a gross oversimplification...There are probably many different genes involved..." (p. 8) Later on he writes, "I believe our genetic predisposition for faith [notice: not "God"] is no accident. It provides us with a sense of purpose beyond ourselves and keeps us from being incapacitated by our dread of mortality." (p. 143) Note also that the book's subtitle declares that "Faith is Hardwired into Our Genes" while on page 211, Hamer declares that we are "Softwired for God."
Hamer's problem with definitions and usage arises because he is trying to take an abstraction such as "spirituality" or "transcendence" or "faith" or a belief in "God" and measure this abstraction with personality tests or by observing broader forms of human behavior. Furthermore he wants to make a useful distinction between religiousness and spirituality, between the extrinsic and intrinsic expression, the former being mostly public, such as church attendance, and the latter mostly private, such as prayer or meditation. Having done this he then wants to find a gene or some genes that code for spirituality. This is like trying to catch the ether in a hairnet.
Nonetheless it goes almost without saying that however ill-defined such abstractions may be, they do in fact refer to something real. A belief in an afterlife, in souls and inherited karma, in gods and poltergeists, heavens and hells, in things mystical and extrasensory, in a reality beyond a purely material and animal existence is universal to all human societies, past and present, and would seem to be as necessary as the very air we breathe. (Gurus, churches and religions exploit this human necessity.)
Consequently it is not so far-fetched to look for the predisposition for such beliefs in our genetic code, genes that have been selected by the evolutionary process. The question remains however, exactly what behavior is it that is selected and found adaptive in an evolutionary sense? Hamer thinks it is some sort of personal transcendence--that is, spirituality as opposed to religion as such (see page 215). However I think there is reason to believe that what is selected is the more profane aspects of religion and spirituality. To put it bluntly, what the genes (interacting with the environment of course) code for are tribalisms such as following a leader and being willing to die for the good of the tribe, and in general following the authority of tribal ways and means, believing what the shaman says, what the priest says, what the ayatollah tells us, and what the documents of the tribe declare as true.
Edward O. Wilson in his book, On Human Nature (1978)--highly recommended, by the way--argued that the ability of the individual to conform to the group dynamics of religion was in itself adaptive. He added, "When the gods are served, the Darwinian fitness of the members of the tribe is the ultimate if unrecognized beneficiary." (opus cited, p. 184)
Still there is a sense in which it is possible to see the genetic predisposition toward faith and religion in a more morally positive sense. Hamer believes that "God genes...provide human beings with an innate sense of optimism." (p. 12) Clearly life must be worth living, and faith provides us with any number of compensations for a hard life: promise of an afterlife, a rebirth to a better station, karmic comeuppance for transgressors, and karmic reward for our perceived good behavior, punishment for sin, etc., are artifacts of faith and are the main tenets of many religions.
However as to the specific gene that Hamer comes to identify, the VMAT2 gene, which influences the flow of monoamines in the brain, it could be said that this gene is not so much the "God gene" as the "dope gene," the gene that helps us to get high. On page 77 he allows that "There might be another 50 genes or more of similar strength."
In addition to Hamer's central argument, there are aspects of this book that are interesting and valuable in themselves. The chapter on "The DNA of the Jews" is absolutely fascinating and gives us a good idea of what is possible by using the changes in either the "y" or "x" chromosomes to trace human migrations and intermarriages.
I also like the distinction that Hamer makes between spirituality and religion. We all know people who are spiritual, but don't go to church (or temple or mosque, etc). And we all know people who attend church regularly but are about as spiritual as hyenas. (I won't mention any White House occupants, past or present!) And it is clear that there are agnostic scientists who are very spiritual persons indeed.
However, the weakest part of the book involves Hamer's attempt to adequately define spirituality and to distinguish it from religion. He calls in the psychology and psychiatric establishments to help out. I don't think they help much. It is a daunting task to even define "God" adequately. In the final analysis he goes with the idea of transcendence. However what we humans want to transcend is our animal nature (and sometimes the evidence of our senses and our experience!). Part of the reason we wear clothes and otherwise cover up while imagining that we have souls and are made in the image of God is to make our animal nature less obvious. For human beings it is not sufficient to be just animals. We are (or should be) spirit as well. Hamer actually declares that "Spirituality...is, in fact, an instinct." (p. 6)
Finally, faith does not require a god. Taoism has "the way," and the Buddha famously turned aside questions about God as being beside the point, while the ineffable God of the Vedas is nothing that a believer in a personal god would recognize at all.