Male, monozygotic twins (six pairs) were repeatedly tested before and after d-amphetamine, l-amphetamine, or placebo administration. Drug effects on cognitive, psychomotor, personality, mood, and pain variables were assessed. Members of a twin pair tended to respond similarly on several tests under placebo conditions, indicating genetic determination of the behavioral variables. In addition, cotwins tended to show similar responses to amphetamine as measured by one test of cognitive function, by several mood and personality variables (hostility, autonomic arousal, friendliness, feelings of tension and loss of control), and tended to have similar plasma levels of both amphetamine isomers. Although shared environmental effects cannot be ruled out, the results are consistent with genetic mediation of a variety of behavioral effects of amphetamines.

Thirty-two pairs of Caucasoid twins, 16 monozygotic (MZ) and 16 dizygotic (DZ) of the same sex, were studied by densitometry in relation to the C-bands of chromosomes 1, 9, 16, and Y. Confirming earlier results, concordance was not absolute among MZ. Estimates of the degree of genetic determination for these traits varied from 0.73 to 0.89 for the autosomes and from 0.86 to 0.95 for the Y. There are now stronger indications that a fraction of the intergeneration variability found in these structures may be real, probably due to mitotic and/or meiotic unequal crossing-over.

The low birthweight of twins compared with singletons is only slightly influenced by the higher congenital abnormality rate in twins, or the increased incidence of proteinuric pre-eclampsia in the mothers. Reduced intakes of energy food or of zinc, copper, and iron do not account for the lower birthweight. The main cause of low birthweight is preterm delivery, and this is more common in monozygotic than dizygotic twin pregnancies, due particularly to premature rupture of the membranes. The type of placentation did not influence the preterm onset of labor. Preterm labor in monozygotic twin pregnancies is associated with a very high boy:girl ratio, but this did not apply in dizygotic twin pregnancies.

Nutritional status of 19 pairs of twins and 17 singleton controls was assessed by measuring dietary intake and selected anthropometric and biochemical parameters. The results indicated no significant variations between first and second twins and singleton controls and between all twins and controls for all parameters studied. Significant variations were found in serum α-globulin and protein and energy intake when male and female twins were compared with male and female controls.