Broadly speaking, there are two interrelated factors that contribute to the development of people's personality, and therefore, to the development of personality disorders. These are: 1) biological factors, meaning people's genetic make-up and temperament, and 2) environmental factors, meaning people's life experiences, particularly early childhood experiences. Although it is simpler to discuss these two factors separately, many experts believe they cannot be understood independently from one another. This is similar to the futility of discussing, "Which came first, the chicken or the egg?"
People's genetics and their early life experiences interact in complex ways to influence the development of their personalities and subsequently, their vulnerability to the development of personality disorders. This interaction between people's biological, genetic dispositions and their environmental, life experiences is often referred to as the "nature-nurture" dynamic.
We know that both nature and nurture play vital roles in the formation of personality disorders. We also know that neither nature nor nurture is capable of creating a personality disorder in isolation from the other. What we don't know is how these forces specifically interact to have their effect on personality development. The study of the personality disorders remains a relatively recent endeavor. As is frequently the case in such situations, more research is needed before we can speak with greater certainty about the precise causes of personality disorders. Thus, our current state of research only enables us to identify factors that are known to influence personality development, but we cannot say with any certainty that these factors cause personality disorders.
It is particularly difficult to talk about the exact causes of personality disorders (taken as a group) because few researchers study all the personality disorders at once. At the present time, the more commonly diagnosed personality disorders, such as the Borderline Personality Disorder, have attracted the lion's share of research attention. Consequently, our knowledge of these more commonly diagnosed disorders is far more advanced than is our knowledge of the less common disorders. Notwithstanding these limitations, we will review some of the scientific evidence that has informed our present understanding of personality development in general, and the more specific development of personality disorders.