In 773 three-generation families of twins, individual probability of nonrighthandedness (NRH) depends significantly on the handedness of that individual's parents. The parents of twins are much more often NRH than are their nontwinbearing siblings. The twins and their siblings apparently inherit the excess liability for NRH shown by their parents. Monozygotic (MZ) pairs are significantly about twice as often concordant (casewise) for handedness as dizygotic (DZ) pairs. Overall, the best-fitting mode of transmission is autosomal, NRH dominant to RH with penetrance of about 50% in the heterozygote, or multifactorial, with heritability of 60–70%. Under the major-gene approach, heterogeneity seems likely, with about one-third of the families showing no NRH outside of the twin sibships. MZ/DZ concordance ratio is about four in those families, suggestive of recessive inheritance. A 1.16-fold excess of NRH in twins compared to their siblings, and a 1.8-fold excess in the second-born members of same-sex discordant pairs, leaves open the possibility that a minor portion of NRH in twins may be secondary to transient hypoxia and/or acidosis.

In a study carried out in an African population in Western Nigeria and a Caucasian population in Aberdeen, Scotland, it was found that DZ twinning rates varied with maternal age and parity, the MZ twinning rate remaining fairly constant. However, women aged 30–34 were found to have the highest rate in Western Nigeria whereas the peak in Aberdeen population occurred in the older age group, 35–39 years. Other factors that influenced DZ twinning rates were maternal height, social class and ethnicity (in the Nigerian population), and illegitimacy (in the Aberdeen population). No significant association was found between twinning and maternal blood groups or season of the year in either of the two populations. An important factor that also influenced twinning in the two populations was the maternal serum FSH level. The levels were much higher in the Nigerian population than in the Aberdeen population. Furthermore, in the Aberdeen population, twin-prone and non-twin-prone women had similar serum FSH levels, whereas the levels were much higher in twin-prone women in the Nigerian population. This finding is consistent with the fact that the Nigerian population has a much higher twinning incidence (approximately 50 per 1,000 maternities) than Aberdeen population (approximately 12 per 1,000 maternities).

Sixteen pairs of term discordant twins (weight discrepancy of more than 20% when the, lighter twin was compared to the heavier) were evaluated at birth. Weight, length, head circumference, anterior fontanel area, and combined ossification of the knee epiphyses were measured, and ponderal index (weight/length ratio) calculated. The most severely growth retarded infants had markedly decreased ossification and larger anterior fontanels.

Eleven pairs had physical measurements at one year of age. With individual exceptions, the lighter twins at birth remained smaller in all dimensions. Despite these persistent differences between twin pairs, the values for length at one year of age were within normal limits for both the heavier and lighter twins. Infants without ossification at birth had a greater incremental linear growth by one year than those infants with ossification.

A total of 71 pairs of like-sexed dizygotic twins were studied, comparing within-pair differences for plasma total, free and esterified cholesterol, and triglyceride with the number of HLA haplotypes the twins had in common. If associations are present between HLA and the blood lipids studied, the twins with no haplotypes in common would be expected to have the largest within-pair mean square, those with two in common the smallest, and those with one in common an intermediate value. No significant differences were found comparing within-pair mean squares for the variables studied.

A statistical index is proposed in order to determine twin zygosity based on weight and height measures and a Bayesian test of hypotheses. Some Roman twin data are analyzed on the basis of the Euclidean distance index, which appears to be the best for an easy calculation and for its genetic interpretation based on the formulation of genetic distance.