Johansson, B., Whitfield, K., Pedersen, N. L., Hofer, S. M., Ahern, F., & McClearn, G. E. (1999). Origins of individual differences in episodic memory in the oldest-old: A population-based study of identical and same-sex fraternal twins aged 80 and above. Journal of Gerontology, 54B, P173-P179.