Temperament includes behavioral traits such as sociability (outgoing or shy), emotionality (easy-going or quick to react), activity level (high or low energy), attention level (focused or easily distracted), and persistence (determined or easily discouraged). These examples represent a spectrum of common characteristics, each of which may be advantageous in certain circumstances. Temperament remains fairly consistent, particularly throughout adulthood.
Similar temperaments within a family may be attributable to shared genetics and to the environment in which an individual is raised. Studies of identical twins (who share 100 percent of their DNA) and their non-twin siblings (who share about 50 percent of their DNA) show that genetics play a large role. Identical twins typically have very similar temperaments when compared with their other siblings. Even identical twins who were raised apart from one another in separate households share such traits.
Scientists estimate that 20 to 60 percent of temperament is determined by genetics. Temperament, however, does not have a clear pattern of inheritance and there are not specific genes that confer specific temperamental traits. Instead, many (perhaps thousands) of common gene variations (polymorphisms) combine to influence individual characteristics of temperament. Other DNA modifications that do not alter DNA sequences (epigenetic changes) also likely contribute to temperament.
Large studies have identified several genes that play a role in temperament. Many of these genes are involved in communication between cells in the brain. Certain gene variations may contribute to particular traits related to temperament. For example, variants in the DRD2 and DRD4 genes have been linked to a desire to seek out new experiences, and KATNAL2 gene variants are associated with self-discipline and carefulness. Variants affecting the PCDH15 and WSCD2 genes are associated with sociability, while some MAOA gene variants may be linked to introversion, particularly in certain environments. Variants in several genes, such as SLC6A4, AGBL2, BAIAP2, CELF4, L3MBTL2, LINGO2, XKR6, ZC3H7B, OLFM4, MEF2C, and TMEM161B contribute to anxiousness or depression.
Environmental factors also play a role in temperament by influencing gene activity. In children raised in an adverse environment (such as one of child abuse and violence), genes that increase the risk of impulsive temperamental characteristics may be turned on (activated). However, a child who grows up in a positive environment (for example a safe and loving home) may have a calmer temperament, in part because a different set of genes is activated.