Does evolutionary psychology make it hard to understand "culture"?

I've been reading Why Beautiful People Have More Daughters over my Christmas vacation. It's similar to Robert Wright's The Moral Animal in that it popularizes evolutionary psychology. But the authors (Alan S. Miller and Satoshi Kanazawa) update Wright's 1994 book by using more recent research to answer a slew of questions about society.

In a future post I'll excerpt some of the more useful insights from the book. For now, I have a complaint with the authors' discussion of "culture."

They start out the book saying that "everyone" realizes that environment, culture, etc. shape human behavior. "[T]here are no serious biological or genetic determinists in science" (pg. 2). The only controversy is whether biology, genes, evolution, etc. also shape human behavior. Of course, the authors, being evolutionary psychologists, are adamant that the answer is "yes." I agree.

But it's not so clear to me that the authors actually recognize that culture shapes human behavior. They say:

Gender socialization helps to accentuate, solidify, perpetuate, and strengthen the innate differences between men and women, but it does not cause or create such differences. In other words, men and women are not different because they are socialized differently; they are socialized differently because they are different. Gender socialization is not the cause of sex differences; it is their consequence. (32) (emphasis in original)

The first sentence in that paragraph sounds relatively nuanced, but the rest of the paragraph eliminates any nuance. Their position seems clear: gender norms are simply not a cause of any differences between men's and women's behavior. It's hard to imagine a more extreme position than that.

Similarly, they say:

[T]he sex differences in behavior, cognition, values, and preferences are largely innate; universal across cultures; and, in many cases, constant across species. If the sex differences were the result of social and cultural practices such as gender socialization, then they should by definition vary by culture and society. In fact, however, in every human society (and among many other species), males on average are more aggressive, violent, and competitive, and females on average are more social, caring, and nurturing. What is constant in every culture and society (sex differences in behavior) cannot be explained by what is variable across cultures and societies (cultural and social practices). A variable cannot explain a constant; only a constant can explain a constant. (32) (emphasis added)

But later in the same chapter, they have this heading:

There Is Only One Human Culture

Um, I thought they just said that anything that results from culture must, "by definition," "vary by culture and society." But no:

[A]ll the cultural differences are on the surface; deep down, at the most fundamental level, all human cultures are essentially the same .... [C]ulture is a universal trait of all human societies. Yes, culture is a cultural universal.

I'm open to hearing out someone's argument that all human culture is the same. And I'm open to the argument that culture is wildly varied; therefore, any traits that are universally present in humans cannot be the result of culture. But is it too much to ask that the same co-authors stick to a consistent position within a given book?

Aside from the internal inconsistency, I'm not convinced that sex differences are so universal that they can't be the result of social norms. One problem with this is: how could you ever conclude that a particular sex difference is universal? Even if you've surveyed every society in the world and found that, for instance, women refrain from serving in the military in all of them, you'd still only be speculating that this will continue to be the case for all time. Of course, women didn't used to serve in the military, but they do now. Many jobs used to be considered out of the question for women, but this is no longer the case. Who's to say that a sex difference that appears universal right now won't crumble in the future?