In environments where sperm from multiple males are competing intra-vaginally for fertilization, cooperation may actually increase the odds of reproductive success. Think of it this way: in species that are promiscuous by nature, sperm from the same male might have a tendency to cooperate instead of compete in order to reduce the odds that sperm from another male will reach the egg first.
Indeed, this is what the Harvard study found: the researchers studied two species of mice, one that is promiscuous by nature (Peromyscus maniculatus) and one that is monogamous (Peromyscus polionotus). In the promiscuous mice (where sperm from multiple males are often competing in the female reproductive tract), their sperm had a tendency to clump together in optimally sized packs that ended up reaching the egg faster. In contrast, in the monogamous mice (where sperm don’t have any external competition), their sperm were less likely to form these optimally sized packs.
If the authors of Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships are correct then humans should have sperm that tend to clump together. If the author of Sex at Dusk: Lifting the Shiny Wrapping from Sex at Dawn is correct then human sperm will tend not to clump together.
So why haven’t the Harvard researchers published the data?