Explanations attributing AN to biological dysfunction lack explanatory coherence. They must invoke multiple independent malfunctions to account for the syndrome. For example, Kaye and colleagues proposed that food refusal is caused by patients’ attempts to reduce serotonin levels, body image distortion and starvation denial are caused by a right hemisphere dysfunction, and that hyperactivity may be due to imbalance between motor area dopamine and serotonin. Such piece-by-piece explanations do not provide specific mechanisms and are silent on why these breakdowns happen together and normalize together with weight recovery.
In contrast, an evolutionary explanation does account for the broad array of AN phenomena, and it is supported by extensive evidence from multiple independent sources. The hypothesis that AN symptoms were selected in the past when starving foragers needed to migrate to find food accounts for AN’s puzzling psychological symptoms: The ability to ignore hunger and move energetically would have helped Pleistocene foragers leave a depleted home range. Self-deception about fat stores could have provided optimism to travel. Self-control, increased pain tolerance, cognitive rigidity, and the associated feature of perfectionism would have helped individuals persist on desperate journeys that few survived.
The hand of natural selection leaves fingerprints in the genes and in the phenotype. Nesse and Williams have argued that natural selection is implicated when a genetic illness has significant incidence and multiple genetic changes affecting the same or overlapping functions. Adaptations look “designed” for a purpose and are often complex, efficient, modular, and universal across different cultures. The complex psychobiological changes of AN are difficult to explain except as the result of specific biochemical mechanisms resulting natural selection. The observations that AN is heritable, has a homogeneous presentation in individuals with only very low weight in common, and symptoms that remit with weight gain all strongly suggest that the symptoms are an evolved response to starvation.