An evaluation of the intrapair relationship for personal attributes in the NHLBI twin study.

We have considered a group of questions which attempt to define the environments of twins in our study both within and between twin pairs. Only 8 of 31 questions on which twins compared themselves to their co-twins were reliable and showed discordance for twin pairs. Comparing discordant twins for cardiovascular risk factors, few significant results were obtained. Cigarette smoking seemed to be most closely related to the environmental differences; but negatively related to achievement in grade school and high school and to which twin was stronger in childhood. Weight at examination appeared to be related to strength in childhood. In addition, there may be a negative relationship between income and blood pressure. One additional question showed MZ twins get together significantly more often than DZ twins. In terms of differnces in environmental covariance, the question "how frequently do you and your twin get together now?" may be a simple but valuable measure of similarity of twins and should be considered for inclusion in twin studies.