An overview of the existing literature suggests that traits similar to BPD are influenced by genes. It is too early to say to what extent BPD is also influenced by genes, but because personality traits generally show a strong genetic influence, this should also be true for BPD. Nonetheless, if the equal-environment assumption were to be violated for MZ and DZ pairs, twin studies may be overestimating genetic effects and hiding the effect of common family environment. The less than-ideal reliability of measurements used in this research may also reduce the effects of genes and common environment while increasing the effects of unique or nonshared environment. The effect of genes on the development of BPD is likely substantial. The effect of common family environment may be close to zero. More studies, large and small, are needed to reach firmer conclusions about the influence of genetics on BPD.