Behavior is influenced by a person's genetic make-up. The debate between behavior being totally defined by the way a person is brought up against that of it being defined by the person's genetic make-up is common called nurture vs. nature.
And both have a contribution to make in the way everyone behaves. As an example two identical twins that have the same genetic code would not make exactly the same decisions when they encounter the same situations.
Yet, scientists have been able to identify genes that influence the production of various hormones, an example of which is dopamine, in our brain. These hormones determine many behavior traits of people. For instance they influence the level of risk people are willing to take under the same circumstances.
More research is being done to identify the role played by other genes in behavior so that people who face problems due to their behavior traits can be helped in a better way.