Critics fault Crabtree for his loose phrasing, sparse proof and reliance on "it stands to reason" arguments. They also disagree with his readings of certain genetic, neurobiological and anthropological studies [sources: Kalinka et al.; Mitchell]. In short, the head bone of contention extends beyond Crabtree's claim of brittle braininess to the very essence of his evidence.
For example, Crabtree dismisses language's role in expanding the frontal cortex and endocranial volume -- two changes vital to the evolution of abstract thought that he says occurred 50,000-500,000 years ago. But Alex Kalinka and his colleagues at the Max Planck Institute argue that language capacity stretches back much further, to when modern humans split off from Neanderthals, which could suggest an early, language-related ballooning of the brain [source: Kalinka et al.].
Mitchell, meanwhile, defends society's role in bolstering brainpower. What Crabtree views as a crèche that coddles the less clever, Mitchell sees as the very crucible from which acumen arises, pressured by the complexities intrinsic in social dealings [source: Mitchell]. Crabtree replies that societal impacts -- including environmental, nutritional, perinatal and educational improvements -- merely offset genetic decline [source: Crabtree].
On that topic, Kalinka and company dismiss Crabtree's XLID-based estimate of 2,000-5,000 intelligence deficiency genes. Here's why: The X chromosome boasts more than its share of brain-expressed genes and virtually bristles with links to neurological genetic disorders. In other words, any estimate based on the X chromosome should skew quite high and therefore offer a poor basis for estimation [source: Kalinka et al.].
Returning to the easily broken heart of the matter, Mitchell attacks Crabtree's idea of delicate intelligence on the ground that it would work only if mutations affected humans not as individuals, but as a species, which isn't the case. If it were, he argues, every trait touching on our fitness would degrade over time as we accumulated one nasty mutation after another (sidebar). Not only does this not occur, says Mitchell, but intellectual deficiency genes (ID) tie into a vast web of other genes whose mutations do not cause ID -- a far more robust system than Crabtree describes [source: Mitchell].
Finally, say critics, competing models of genetic development strongly argue for intellectual resilience. Intelligence might not derive from genes that specifically code for it, they say, but rather arise as a side effect of general fitness, a hitchhiker on the coattails of other selected-for traits (or vice versa). Such a model would help explain why intelligence often goes hand-in-hand with various physical and mental health characteristics, including cardiovascular and psychiatric diseases [sources: Deary; Kalinka et al.; Mitchell; Yeo et al.].
In the end, Crabtree himself views his fragile intellect idea as a useful hypothesis, one he admits is in need of greater study and substantiation [source: Crabtree]. Meanwhile, the debate itself underlines just how complex our genetic dance might be -- and how much work remains to be done.