Males are more aggressive and testosterone has been linked with crimes such as murder and rape. Testosterone levels peak from puberty to early 20's, which correlates with highest crime rates in males
Criminals were physically different from non-criminals and could be identified by distinct features such as high cheekbones, long arms, prominent eyebrow arches
Criminals were seen as atavistic, throwbacks to an earlier, primitive stage of evolution, pre-social, unable to control impulses and had a reduced sensitivity to pain
Jacob's study found an abnormality of sex chromosomes, with an extra Y chromosome, known as XYY syndrome, in men with this tendency to be very tall, well-built, immature, with low intelligence and a strong tendency to commit motiveless property crimes and be potentially violent
Twin studies have shown that if one twin has a 'criminal gene', the other twin is likely to also have it. Adoption studies have shown that adopted children's criminality is more similar to their biological parents than their adoptive parents, supporting a genetic explanation