Although he says that his ideas arise from anthropology and neurobiology, Crabtree's argument mostly comes down to genetics.
Here's the gist: We all know that genes are little DNA snippets that perform many vital functions in building this biological contraption known as "us," including passing down traits from our parents. Each gene type comes in several flavors called alleles. For example, a gene (one of many) related to height might code for either tallness or shortness. We inherit half of our genes from each parent, and some dominant alleles, such as brown eyes, tend to, well, dominate over recessive ones, like blue eyes.
Sometimes an allele undergoes an abrupt, random change called a mutation. Somatic mutations affect a single organism -- you, for example -- while germ-line mutations can be passed on to offspring. Although most mutations are harmful, in rare cases, one can increase a creature's fitness to live on and breed. Hence, mutation is the engine that drives evolution and natural selection.
Based on studies of intellectual deficiencies linked to genes on the X chromosome (or XLIDs such as fragile X syndrome), Crabtree estimates that at least 10 percent of human genes participate in intelligence and believes that every single one of them plays a vital role in intellectual and emotional function. Intelligence, for Crabtree, is a genetic house of cards, and a bad mutation in even one gene can bring the whole affair tumbling down [sources: Crabtree; Crabtree].
To make matters worse, intelligence is particularly susceptible to harmful mutations and is not readily passed down to children, says the geneticist, which means you need a strong selective force in play if you want to keep your bean keen from generation to generation. Between 50,000 and 500,000 years ago, he argues, the unrelenting struggle for survival put hunter-gatherers under pressure aplenty, but more stable agricultural societies selected for traits that better met the needs of their way of life, such as resistance to the diseases it gave rise to. As for modern society, its various safety nets, though desirable, enable people across the intelligence spectrum to breed. Crabtree offers no evidence to back up these assertions [sources: Crabtree; Crabtree; Mitchell].
In short, intellect and emotional maturity are as fragile as a dream, and we are in for a rude awakening.
Crabtree's hypothesis appears to embrace all flavors of intelligence and emotional development. For example, he includes the "intuitive ... non-verbal comprehension of things such as the aerodynamics and gyroscopic stabilization of a spear while hunting" [source: Crabtree]. This strategy makes his claim hard to prove or refute. After all, other factors involved in survival, such as coordination, training or social qualities, might also map to intelligence -- or fail to, depending on which definition better supports Crabtree's thesis.
This vagueness is part of the problem. Scientists, of course, need to carefully define their terms, specify their problems and outline the criteria by which further research might support or refute their hypotheses. On these grounds, and others, Crabtree's critics take him to task.