Inferring 'types' from external appearance has led science down some unfortunate roads.

Since ancient times, people have attempted to link physical appearance to personality. Aristotle said, "It is possible to judge men's character from their physical appearance, if one grants that body and soul change together in all natural affections".

Such thinking underpinned physiognomy - the study of a person's facial features - and phrenology, drawing inferences from head shape.

German physician Franz Joseph Gall (1758-1828), the founding father of phrenology, believed that the brain was made up of 27 individual 'organs' that created one's personality. Each bump or indentation in a patient's skull reflected the extent to which the person used that organ, thus indicating character. In the 1820s-1840s, people used phrenology to assess prospective marriage partners and job applicants.

The advent of photography in the mid-19th century - and with it the police 'mugshot' - kept physiognomy in the public eye. Criminologists such as Cesare Lombroso (1836-1909) studied criminals' photographs and derived their 'typical' facial characteristics. Sir Francis Galton (1822-1911) used 'composite portraiture', an early attempt at morphing individual images, in the hope of identifying typical criminal faces. But average criminal faces just looked - average.

Galton was a pioneer in the study of human variation - physical and mental. He collected vast amounts of data and invented new statistical approaches to measure human differences, creating the field of biometrics. A cousin of Darwin, he was fascinated by inheritance and the relative importance of family history and environment - indeed, it was Galton who coined the phrase 'nature versus nurture'. He also created the field of eugenics, which aimed to improve human society by selective breeding.

Phrenology and physiognomy have a very sorry history. Applied in a racial context, they influenced the Nazis' thinking on Aryan racial superiority. And in the 1930s Belgian colonial authorities in Rwanda used phrenology as evidence of Tutsi superiority over Hutus, paving the way for the Rwandan genocide of 1994.