But identical twins are usually raised by the same mother in the same home making their environment rather similar. This shows that even in similar environments, differences will still arise regardless of their genetic constitution, i.e. minor variations will result in large differences in phenotype regardless of genotype. Human behaviour is a complex interaction between genes and the environment. In its more advanced forms, behaviour is not determined by genes. For example, the most common behaviour found in adults of any race is the use of language, whereas in newborns, language must be taught.
Primitive behaviour is more likely to be genetically programmed into an individual.
For example, the sucking and grasping reflexes of a newborn infant are present even in the womb, before any interaction with the environment is possible. These behavioural traits may involve reflex arcs present only in newborns because after some time, the individuals no longer behave in this manner. The majority of behavioural traits are acquired later in the development of an individual. Most people go through very strange behavioural changes in the course of their development, especially during puberty and sometimes towards their old age. This may be due to changes in the levels of hormones such as testosterone in men, oestrogen and progesterone in women.