Humanist psychologists would argue against the idea of determinism, claiming that humans have self-determinism and free will. Furthermore, there is evidence to support this claim. Identical twin studies typically find an 80% similarity in intelligence scores and a 40% similarity in the likelihood of depression. However, as identical twins share 100% of their genes, these results suggest that 20% is caused by another (environmental) factors. This demonstrates that biological determinism is unable to explain any particular behaviour, in this case, depression and intelligence. The same evidence indicates that no behaviour is completely environmentally determined. If identical twins only show an 80% likeliness of intelligence, it is therefore, assumed that only 20% is caused by the environment, highlighting a limitation with biological and environmental determinism.