Extroversion or introversion may be an inherited dimension of personality. (Healy, Jane M., PhD. Your Child’s Growing Mind. p 8-10. NY: Doubleday, 1987, 1989.)

Twin studies: there appears to be a genetic component related to extraversion and introversion. In the nature versus nurture controversy, shared family environment appears to be far less important than individual environmental factors that are not shared between siblings. (Auke Tellegen, David T Lykken, Thomas J. Bouchard, Jr., Kimberly J. Wilcox, Nancy L Segal, Stephen Rich (1988). "Personality Similarity in Twins Reared Apart and Together."  Journal of Personality and Social Psychology, Vol. 54, no. 6. 1031–1039.)