Consider a man who has 3 digits on his left hand. What caused this? Well, he was in a car accident, and his hand was severely injured. That might be all of the cause that a person is interested in knowing; perhaps someone only wanted to know if there was an interesting story behind the condition of the man's hand, and perhaps that someone doesn't find car accidents interesting.
Others might feel that this answer is insufficient. Why did the accident happen? Why did he suffer this particular injury, and why was it of the severity it was? There could be many causal factors; a slippery road, poorly marked intersections, inefficient brakes, the man's inattention due to drink or a cell phone or fatigue, and so forth. Which of these factors a particularly questioner will be interested in will vary; an engineer may be particularly interested in the brakes, while a city planner might be interested in the design of the intersection and someone in law enforcement might be most interested in the possible drinking. Why the accident resulted in that particular injury also has causes; aspects of the design of the car, as well as circumstances such as the speed of the two vehicles involved in the collision. Obviously, the speed would be affected by the causal factors identified as relevant to why the accident happened; slightly better brakes, say, might not have prevented the accident, but might have reduced the speed the car was traveling and so made it less severe.
There are also more distant factors. It wouldn't be remarkable that the man had 3 digits on his left hand were it not for the fact that humans normally have five digits. Presumably, the explanation for that must come from biology. So far as I know, the explanation from biology for this particular phenomenon relies heavily on chance and/or unknown factors (anyone who knows better is welcome to correct me). The pattern of four limbs and five digits per limb was established extremely early in the evolution of vertebrates, and it is quite unclear whether this model was more advantageous than other arrangements which also appeared, or if it was merely the only pattern that vertebrates hit on in the early stages (selection can only operate on features which happen to turn up). In some animals, mutations which varied the development of digits were selected for in such a way that the growth of some of the digits was eventually completely eliminated (hoofed mammals, for example). In humans and their ancestors, no mutation varying the number of digits ever proved advantageous enough to be selected for, and so humans still have the pattern of their distant vertebrate ancestors.
Genetic patterns selected for in our ancestors would no doubt also turn out to be involved in patterns of behavior of engineers who build cars, drivers who drive them, and city planners who lay out intersections. Prima facie, these factors are likely to be enormously complex, and I am not aware of even the boldest of evolutionary psychologists proposing that a specific genetic adaptation tending to produce particular intersection designs has been discovered. Cultural factors, likely to mostly be more recent than the genetic factors, also no doubt contribute to the behaviors in question.
Even more distantly, had plants which produce oxygen not evolved, then humans would certainly not have evolved, and arguably beings with sufficiently intense metabolisms to engage in the wide variety of activities pursued by humans and other modern mammals might not have evolved. The particular distribution of chemicals on the surface of the earth was a causal contributor to the chemical composition of early life, and so a contributor to how later life turned out. There are causes studied by cosmologists which brought about the formation of a planet with the properties Earth has. To return to the more recent past again, the earth's meteorology and geology of course cased the road to be icy in that place and at that time, and the causes of the local conditions also extend back in the past all the way back to cosmology again.
The philosophical analysis of causation has long struggled with the problem of picking out the causes of events partly because any very specific analysis of causes faces endless counter-examples, while more permissive accounts very quickly end up identifying the entire prior history of the world up to the event as causing the event. I tend to favor the solution to these issues which says that what counts as a cause is determined by context, by what those seeking the cause are interested in. The flexibility of the approach makes it immune to counter-examples, but it also has the disadvantage of not actually telling us much about causes, and of making it appear that the question of what cause is operative is not really fundamental, since the same answer can be right or wrong depending on circumstances. I find this to be preferable, however, to accounts which try to make causation an objective matter, since they inevitably end up producing very problematic accounts of causes.
Palmer and Thornhill are not philosophers, so perhaps it is not surprising that they appear unaware of any philosophical debates concerning causation. However, they make some very strong and problematic statements about causation, which they at least treat as essential to their overall argument.
Specifically, they distinguish between what they call "proximate" causes, and what they call "ultimate" causes, for human behavior. Ultimate causes for Palmer and Thornhill are those which appeal to selection pressures applying to the underlying mechanisms involved in the behavior. It appears from their discussion that selection pressures acting on genes coding for particular mechanisms are the ultimate causes they have in mind. Proximate causes are those less distant from the behavior itself, and are viewed by Palmer and Thornhill as triggers activating the genes which have come about due to ultimate causes.
"Ultimate" means "final," and so the name seems to be chosen to refer to the way in which the ultimate causes they discuss are the last in a sequence tracing back through history. But of course they are hardly final in reality; as mentioned above, causes can be traced back further. "Ultimate" is also commonly used to mean "most fundamental," and by extension "most important." Given their discussion, it is hard to avoid the impression that they consider their ultimate causes to be ultimate in this last sense, though it is hard to see why. It is hard to escape the impression that they really think they investigate causes which are somehow ultimate in the first sense, that is final, and that they think this gives reason to think their causes are ultimate in the last sense, that they are the most important. But even if they weren't just wrong about their causes being ultimate in the first sense, there is no logical connection between the first sense and the last sense; in any particular case it would need to be shown that the most distant cause really is the most important.
This is not merely a technical quibble. They end A Natural History of Rape by providing some (incredibly brief) proposals for reducing the incidence of rape, and stating that their proposals are based on their supposedly having discovered the ultimate causes of rape, and that they are for that reason superior to alternate proposals which have ignored the ultimate causes of rape.
This is doubly confused. First, it relies on the notion that "ultimate" means "most important;" obviously, endless causal factors must be ignored by those attempting to design a policy, just because there are so many more causal factors than it would be practical to consider. Second, it assumes that "most important" has a univocal meaning for scientists investigating the development of human psychology and policy-makers seeking to deal with present features of human psychology. But the proximate causes, in T&Ps terminology, are incredibly complicated and diverse, and not very well understood. Until there is a detailed theory of the proximate causes, surely the best way to design policy is to base it on empirical evidence concerning which policies are effective. Ultimate causes may help develop a theory of proximate causes, and even an undeveloped theory of proximate causes may provide ideas for policies worth testing, but even a very highly developed theory of the full causal mechanisms (which we are obviously nowhere near) would still require testing of the particular proposals, and absent such a full theory, what hints and snatches we have of any of the causes of a behavior can only be secondary considerations to policy makers.
Perhaps this is why Palmer and Thornhill's recommendations are so minimal and banal; in reality, nothing much about which policies are best follows from their theory. Suppose, as they say, the inclination to rape is connected to the desire to reproduce, such that preferred rape victims will tend to be similar to the preferred objects of lust generally. It nonetheless could turn out that the feminist proposals T&P mock are more effective at discouraging men from raping than their proposals.
Just off of the top of my head, here's one way that could be the case. What we lust after is obviously influenced by endless environmental factors; various associations produce fetishes, and various trained inhibitions are sometimes successful in suppressing arousal in some cases. Perhaps associating rape with violence rather than sex, and attempting to inculcate stronger inhibitions against violence, will serve to make rape less arousing for men. Then, if rape is motivated primarily by lust as T&P propose, the reduction in arousal from rape would reduce the motivation for rape. And that's just one mechanism which could make the feminist policy effective; I'm sure there are endless others which could be operative. Knowing whether any of them are operative, and so whether the feminist approaches are helpful, requires vastly more research into the effectiveness of various policies; it is not settled by the "ultimate causes" T&P describe.
It is worth mentioning that such research as there is into variation between different cultures with respect to rape frequency suggests that greater economic and social power for women in a society is one of the factors most strongly correlated with reduced rape frequency. This at least gives some reason for thinking that the feminists T&P criticize are right to focus on issues of power, even if their theories about exact mechanisms are often no better justified than those offered by T&P.