Sociobiology is the craniometry of the future.
Actually, the problem is rooted in the influence of Darwinism in the late 19th century, which actually had much more of an influence on Marxism than is often recognized. In other less subtle thinkers, it went overboard. For example, Kautsky was deeply influenced by Herbert Spencer before he ever read Marx. Plekhanov was also affected by Darwinian thought in a negative way. He also wrote something called the Materialist Conception of History, which made many of the same mistakes as Kautsky.