Science like that makes me vomit. Figuratively.
First interjection: How can anyone with any knowledge of biology even write something like this?
Stepchildren do not carry any of the genes of the stepparents, so there is absolutely no evolutionary reason for stepparents to love, care for and invest in their stepchildren.Wrong. On the semantic level: Stepparents share ALL genes with the stepchildren. All humans have all the same genes*. A gene can be approximately thought of as a sequence of DNA that codes for a particular protein. Many genes then come in different versions in the human population - they are called alleles. One famous example is the gene coding for Hemoglobin for which there are two versions, one of which makes sickle-celled Hemoglobin. This may seem like a minor mistake, and I am sure that Kanazawa knows the difference.
But then, what sense does it really make to say that "stepchildren do not carry any of the genes of the stepparents"? Suppose we here substitute "allele" for "gene", is it then true? No, it is not at all. It's not just Kanazawa, of course. It is very common to hear people saying "each parent shares 50% of their genes with each child." But it's bogus, because we are all related, and there are many genes for which two random people will have the same allele. I am not saying that therefore parents aren't more like their children genetically than any random person - parents and children look alike for genetic reasons, of course. But it remains that the statement as given is factually false. Let me just remind you that when people speak of the genetic relatedness of humans and chimpanzees, then we are all of a sudden 98% (or whatever) identical. Taken together, these statements of course make no sense. Apples and oranges.
Second interjection: It does emphatically not follow that there is no reason to love, care for, or invest in children that one is not the parent of. Not even in evolutionary terms. Humans cooperate, and in evolutionary terms it potentially benefits a parent to care for stepchildren in many ways. How about protecting each other? Stepchildren caring for children, for example. I know the story of male lions taking over a pride and killing the cubs that aren't theirs, so that the females come into estrus quicker, and in the light of this, the hypothesis that parents are more likely to kill their stepchildren than their own children does make sense. But always watch out for "making sense". My high school physics teacher liked to tell how "common sense" had been put into an urn and shelved away so that it may not delude the scientist.
Third interjection: The Swedish team examined data that suggest that stepparents are from the beginning more likely to be violent, and Kanazawa readily admits that this could explain the "Cinderella effect". Single mothers perhaps aren't the kind of women in the highest demand, and neither are violent males, so it seems like a good hypothesis that stepfathers are more likely to be violent than the average male. And this is the problem: If one evolutionary psychology hypothesis - trusted and expounded for many years - cannot explain the data, then it is dead easy to come up with another that can. And that's the danger of how evolutionary psychologists practice science. It's not that studying human behavior is not good science (it is). It's not that the link between genetics and behavior is bogus (it isn't). It's the readiness to attribute an evolutionary explanation to psychological data that is problematic. Not because such explanations can't be true (and in some cases they likely are), but because of the ease and certainty with which "common sense" is applied. This sordid affair is a perfect case in point.
The only major weakness of Temrin et al.’s study, which the authors themselves openly acknowledge, is their extremely small sample, taken from one small nation. There just aren’t many homicides in Sweden, child homicides or otherwise. So their findings must be replicated, with larger samples and in other societies. But, at the very least, their paper has begun to throw one of the foundational principles of evolutionary psychology into possible doubt. In my experience, this is the first and (so far) the only study ever to do so. If their findings are replicated, and if their explanation for the greater risk of homicide faced by stepchildren is true, then Hans Temrin and his colleagues have secured their places in the Evolutionary Psychology Hall of Fame for their act of successful academic regicide.Kanazawa writes this apparently without any kind of reflection, which I think is appalling.
* There may be super rare cases where a whole gene is deleted or added in offspring.
Jerry Coyne writes about human-chimp genetic similarity in Why Evolution is True. His estimate is that at least 80 percent of genes differ in at least one codon coding for amino acids: