Suggested criminals were genetic throwbacks, a primitive sub species biologically different to non criminals. They lacked evolutionary development, and were savage and untamed, inevitably turning to crime.
He argued the form could be identified with physiological markers, these include:
- strong jaw, high cheekbones, dark skin, curly hair, facial asymmetry.
He categorised particular types of criminal in terms of feature, eg murderers had curly hair and long ears, and sexual deviants had swollen fleshy lips and glinting eyes.
Ectomorphs (skinny) - cerebrotonic personality, introverted and sensitive
Endomorphs (fat) - viscerotonic personality, relaxed and extroverted
Mesomorphs (muscular) - somotonic personality, assertive and aggressive - most likely to be criminals.