This chapter reviews genetic theory and research on trait loneliness. Using evolutionary psychology, the chapter provides an explanation of why trait loneliness may be the result of selective pressure and therefore be present in humans through genetic influence. The review includes an overview of how loneliness is a product of upbringing and social circumstances as well because of genetic influence. A description of behavioral genetics principles and techniques explains how researchers can estimate the proportion of observed variance which is due to genetics and the environment. A summary of the key findings related to trait loneliness suggests that loneliness has a heritable component and that evolutionary psychology can explain why loneliness serves a role in survival.