Not unlike Michael Behe's book "Darwin's Black Box", one doesn't need to be a specialist in the domain to recognize the logical errors in the arguments presented - no matter how hard the authors attempt to substantiate their position with diagrams, and serious looking statistical analysis.
In many ways, Dr. Whitehead falls into precisely the same trap that Behe did - he's so convinced of the rightness of his argument that he cannot see or recognize the glaring holes in his interpretation of the data.
Consider the following assertion in Chapter 1:
The implications of “many genes” for homosexuality would reflect what happened with the mice, or fruitflies: the typical genetic pattern would be a gradual change in the family over about 30 generations from heterosexuality through bisexuality toward homosexuality - a few percent with each generation. Similarly,homosexuality would only slowly disappear in the descendants (if any) of a homosexual person. Any other proposed mechanism is highly speculative and runs against the known evidence.
The flaws in this argument are many. First of all, it makes the incorrect assumption that evolution would have to progress through bisexual variations to arrive at a homosexual variation. There is absolutely no reason to suspect that this is the case at all, in fact the evidence overall could easily be read as implying that heterosexuality, bisexuality and homosexuality occur naturally and concurrently. This claim implies a determinism that in fact evolutionary theory does not actually reflect.
Similarly, Whitehead asserts that in such a scenario, homosexuality should die out, and yet it clearly does not. Therefore, argues Whitehead, homosexuality could not possibly be rooted in genetics. Again, this makes the false supposition that evolution is deterministic, and further that because a homosexual is less likely to pass their genes on to offspring, that it should die out - more of the "evolution is deterministic" line of thinking. However, it fails to take into account that there may be other reasons why the patterns that result in homosexuality persisting through many generations.
Further, even though we have sequenced the human genome, we should not be so naive as to believe that we have anywhere near a complete understanding of how the genetic attributes we can now describe respond to the surprisingly complex organic chemistry that fills our bodies and makes us tick. We have but begun to explore those very questions.
I'll skip ahead to Chapter 3 for now. Chapter 3 is where the author's bias and agenda is clearly stated:
We all tend to take our heterosexuality for granted as if it just happens. But it seems to develop slowly and steadily and to consolidate over about two decades - through clearly defined and documented stages. Psychologists are in broad agreement about the general stages of heterosexual development and unanimous about one thing: heterosexual orientation is not genetically determined.
This is a strange assertion, and one that seems to be quite at odds with the consensus statements from the APA on homosexuality:
What causes a person to have a particular sexual orientation?
There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles; most people experience little or no sense of choice about their sexual orientation.
This is subtly different from what Whitehead states. Whitehead conveniently leaves out is that the research is inconclusive with respect to causality. The lack of concrete evidence either way should not be interpreted as negation of the idea that there are biological factors involved.
In Chapter 5 (no I'm not deliberately skipping even numbered chapters, but Chapters 2 and 4 really don't say anything significant), Whitehead takes a stab at trying to explain the gender and sexual orientation of Intersex people.
Ironically, his interpretation of Money et. al. falls into almost exactly the same error that Money himself made - namely he confuses gender identity with social gender.
Social gender is largely learned through experience. It builds on gender identity to some degree. If we didn't learn it through experiences, then by what purpose would the natural segregation of male and female children in school years serve? Further, if gender was purely socialization, then the outcome of Money's experiment involving John Reimer would have been dramatically different.
Money's work with John Reimer actually can be understood as validating the narrative of many transsexuals, who almost universally claim that they felt "like the opposite sex" from a very young age (often well before any sense of gender is supposed to be understood by the child) - and transsexuals move towards transition with a surprising degree of persistence - seemingly without swerving once they start to understand themselves.
Whitehead, however, goes on to argue that because many Intersex people choose to remain in the gender role that they were raised in, that gender identity, and correspondingly sexual identity are in fact primarily learned.
Unfortunately, Whitehead has made a serious error in his theoretical construct in making such an argument. Instead of expanding his interpretation to encompass the percentage of Intersex people who do choose to transition to a different gender role as adults, he effectively argues that their story is not relevant and discards that evidence.
There is a fundamental construct out of Mathematics that Whitehead has clearly ignored or misunderstood - namely that of completeness. Mathematical completeness has a strong definition, but the principle applies to scientific theory as well. A theory that fails to encompass the breadth of the available evidence is either in need of revision, or it suffers from logical inconsistencies, and this is where Whitehead's arguments begins to fall apart.
Whitehead wants his reader to be convinced that in the absence of concrete proof of biological causality that sexual identity and behaviour are therefore learned. If something can be learned, it can obviously be "unlearned" or changed, right?
Well ... perhaps that is the case - after all transsexuals learn the social aspects of their chosen gender, often in the face of having transitioned later in life.
However, that does not explain in the least their stated motives for choosing to transition (or, in the case of some, making the choice not to transition).
For much of the rest of his book, Whitehead spends his time expounding on how various lines of investigation have "not turned up any conclusive evidence" in building his argument that sexual orientation is primarily learned behaviour.
Right now, based on much of what Zoe Brain keeps digging up, I think the interesting work is not going on in the causality of sexual identity, but in understanding gender.
The more of this evidence that gets published, the more convinced I become that to assume that we must all try to be heterosexual is deeply flawed. In no other respect to we expect people to fit into absolute categories. Even handedness is mixed - few people are absolutely left or right handed. I'm strongly left-handed myself, but even there, I find that there are things that I do right handed. There's no absolutes in life, and it seems to me that where we are talking about sexual or gender identity, we should not be attempting to impose some kind of absolute models either.
I think that the notion of gender, and sexual identities as occurring along a spectrum of behaviour, as discussed in this essay series is a more reasonable notion than looking at it as if it is all learned, or all innate. It is far more likely that it is actually a mix of factors, and none of us should assume that there is an absolute of any sort at play.
Whitehead's book is essentially a piece of apologetics for the ex-gay lobby. It depends on the classic logical fallacy that the absence of conclusive evidence is equivalent to negation. This is no different than the classic "gaps in the fossil record" arguments against evolution theory - it fails to prove anything, and does not acknowledge that the evidence is gradually getting filled in. Further, because it requires us to discard information in order to hold together, Whitehead's work is clearly based on a weak foundation.