Out of all of the sciences, psychology hold the most controversy in the nature vs. nurture debate. This is the debate surrounding whether behavioural traits are created and influenced by one's genes or one's environment.
There are two classical sides in this debate. Nativists believe that everything that constitutes a person comes at least primarily, if not entirely from their genes. Empiricists post that people’s behaviours are almost entirely influenced by their environment.
In reality, genes interact with the environment at every level. This means that all behaviour results from both nature and nurture. Although historically the debate has been more binary, most psychologists agree that both genetics and environment influence all behaviour on some level. Since this shift in the study of psychology, the direction of that nature/nurture debate has gone from 'is behaviour dictated by genes or environment' to 'what is the relationship between genes and environment in expression behavioural traits?' Many new fields have emerged that study this relationship.
Behavioural geneticists study the relationship between genes and behaviour. Although genes do not have a direct influence on behaviour, they guide cells to generate proteins that cause cells to form chemicals in the body that are related to behaviour, such as neurotransmitters.
Most behaviours are found to be caused by an interaction of multiple genes with the environment. Most disorders with genetic traits are polygenic - meaning that they are influenced by more than one gene.
Epigenetics is the study of heritable changes that occur without a change in DNA sequence. The environment can actually influence how genes behave and express themselves.
A common example used in the nature/nurture debate is that of twins. If two identical twin grow up separately from each other, they can grow up to look different due to their different lifestyles, even though their DNA is still identical. The epi genome does not change the DNA but decides whether or how much some genes are expressed in different cells in the body. Environmental factors that can do this include stress, diet, behavior and toxins that can activate chemical switches to regulate gene expression.
These graphs illustrate the field of psychology's shift concerning the nature vs. nurture debate.