The hypothesis of seasonality of twin births was investigated in two important maternity hospitals in the State of Sāo Paulo, Brazil. The study included 1,386 twin births that occurred among 154,699 deliveries from 1984 to 1993. No evidence of seasonality has been detected either for the twin birth rate considered as a whole or for dizygotic twinning rate. The distributions of these rates fitted well sinusoidal regression curves but the cyclic trend did not correspond to any specific season.

During the years 1964 to 1971 a nationally representative sample of MZ and DZ twins and controls was followed through the Swedish compulsory school. The main purpose was to study physical and mental growth during puberty as well as heredity-environment influences on these growth processes.

After 20 years a follow-up has been made of this sample with the purpose of investigating heredity-environment influences on life situation and self reported health at mid-life in relation to background factors collected during adolescence. 43 pairs of MZ twins, 90 pairs of DZ same sex and opposite sex twins as well 322 controls agreed to participate. A questionnaire was sent out to this group dealing with their present life situation such as family structure, economy, education and occupation. Other areas of interest were self reported health and ability to cope with their present life situation. The disadvantages found for MZ female twins at adolescence seem to persist at mid-life and ratings of school adjustment were related to coping ability as grown ups. This first report presents comparisons between twins and controls as well as males and females. To estimate heredity influences intraclass correlations for the twin pairs are calculated and for coping ability such influences seem to be quite conspicuous. Sex specific factors seem to operate regarding coping ability and satisfaction in educational choice and level. Generally, the results indicate some advantages for the males regarding self reported life situation and health. Possible reasons for such sex differences are discussed. This study has been supported by grants from the Swedish Council for Social Research.

This paper reviews four economic studies of aspects of earnings and schooling conducted by the authors using data from the Australian Twin Register. First, estimates of the economic returns to schooling made using fixed effects and selection effects regression models incorporating an instrumental variables approach to correct for measurement error in self-reported schooling levels are examined. The finding is that up to 30 per cent of the estimated return to schooling may be due to family effects and the remainder to pure educational effects. Second, comparisons are made between the economic model of Ashenfelter and Krueger (1994) and that of DeFries and Fulker (1985) and the results obtained from each are shown to be similar. Third, gender differences in returns to schooling are estimated and family effects are found to be a more important influence in the case of males. Fourth, the influence of family effects on educational attainments is considered and it is found that around one-half of educational attainment is accounted for by genetic inheritance and up to another quarter due to shared environment effects.

A high frequency of twin births has been observed in Linha São Pedro, a small settlement which belongs to the city of Cândido Godói, located 524 km Northwest from Porto Alegre, Rio Grande do Sul, Brazil, in an ethnically homogeneous population of German descent restricted to a small geographic region. From 1990 to 1994, the proportion of twin births in Linha São Pedro was 10%, significantly higher than the 1.8% rate for the state of Rio Grande do Sul as a whole. Genealogical analysis showed a high recurrence of multiple births within families, as well as a high level of inbreeding in the community. Zygosity data indicated that 9 of the 17 pairs of twins studied (53%) were dizygotic. No external environmental factors were detected that could be influencing the appearance of this characteristic. This preliminary investigation confirmed the presumed existence of a high twinning rate in the community. The high familial recurrence and the high inbreeding rate suggests the presence of genetic twinning factors. Complementary studies of twins that have yet to be evaluated and the search for additional risk factors, as well as linkage studies, should contribute to a further understanding of the biological factors related to twin births in the human species.

Twins within pairs often have different weights at birth. A difference of 15% or greater is defined as discordance for weight and is considered to place one or both infants at risk. Temperament differences had been found in the neonatal period for full-term discordant cotwins, but not for preterm discordant cotwins, suggesting that continued gestation for discordant twins was a risk variable for early behavior. 30 pairs of fullterm and 17 pairs of preterm discordant pairs were followed at 6, 9, 12, 18, 24, and 30 months of age. Group differences were observed for the longitudinal maintenance of cotwin discordance in physical measures, with preterm cotwins becoming more like each other. In laboratory assessments, temperament differences no longer were observed between the larger and smaller cotwins. Questionnaires indicated that mothers generally did not differentiate between their larger and smaller cotwin children in temperament ratings, except for ratings of mood for the fullterm pairs. Thus, emotionality was the only temperament dimension that differentiated between the fullterm discordant twins both in the neonatal period and at later ages. In the main, it was concluded that the fullterm discordant twins overcame the adverse in-utero influences on early behavioral development.

This study falls in the areas of both differential psychology and twin psychology. Using the EFT and the WCST (computerized version), we examined 11 MZFF pairs between 18 and 35 years of age. The aim was to establish the genetic and/or environmental determination of global-analytical cognitive style as well as some characteristics of conceptualization linked to field dependence. The research strategy consisted of introducing three other groups of the same size to control the weight of environmental factors different from those determined by subject selection. The results seem to support the hypothesis of genetic determination of field dependence of the MZFFs, probably linked to the XX chromosome combination. The “couple effect” and the attitude of parents and others toward two identical female subjects may contribute to full expression of the genome. The characteristics of conceptualization revealed by the WCST show that MZFFs persevere in errors typical of a global approach to experience.

Population studies on human twinning are scarce in Argentina. In order to analyze frequencies and certain maternal and neonatal variables related to twin births, we studied a series of 69.678 consecutive newborns with 500 g of weight and over, which occurred at a public hospital in the province Buenos Aires, during 14 years (1982-1995). The frequency of twin births (10 per 1000 deliveries) and sex ratio were similar to other studies reported in Caucasian population. Maternal age and order of gravity/parity were positively correlated with twinning rates, more markedly so in dissimilar sex-pairs. Stillbirths and neonatal deaths were more frequent in twins than in singletons, but less frequent when comparing groups of same weight. Congenital malformations were not found to be significantly more frequent in twins than in the total newborn population. However, their occurrence, predominantly in like-sexed pairs and the concordance for defect type in doubly affected same-sex pairs, suggests that monozygotic twinning carries an increased risk for malformation.