Fetal growth is influenced by various determinants, with males being heavier than females. It was recently suggested that female birthweight tends to be increased by the presence of a male co-twin. We evaluated this hypothesis in the Israeli population. We reviewed a retrospective population database of the 1993–1998 Israeli Birth Registry. We compared male birthweight in the male-male and malefemale combination sets and female birthweight in the femalefemale and male-female combination sets. We compared the mean birthweight of males and females in three groups of total twin birthweight. The sample of 12,686 sets included 50.6% males. The male-male combination was found in 31.3% sets, female-female in 38.7% and unlike-sex combination in 30.0%. A total twin birthweight of less than 3000g was found in 7.2% sets, of 3001–5000g in 50.7% sets, and of over 5000g in 42.1% sets. The mean male birthweight was 2398 ± 602 g and the mean female birthweight was 2316 ± 566g, p < .0001 (95% CI of 67, 96). The mean birthweight of males in the malemale combination was significantly lower than in the unlike-sex combination, p < .0001, in total birthweights over 3000g. No significant difference was found between the mean birthweight of females in the female-female and in the unlike-sex combinations in all total birthweight strata. Our findings do not substantiate a male-related inter-twin influence on female birthweight. Population differences might underlie the opposing observations in the literature.

This population based study compares the rates of multiple births in the 1990s in four hospitals of different socioeconomic levels. It is well known that women from higher socioeconomic groups have easiest access to infertility therapies because of greater financial resources. The hospital of lower socioeconomic level presented multiple birth rates of approximately 8 per thousand during the decade, which may be considered as the natural one. The other three hospitals presented increased rates that were positively correlated to socioeconomic level. This increase occurred mainly due to dizygotic twins and to triplets and was as high as 4.8 per thousand in 1999. Maternal age was also positively correlated to socioeconomic level for singletons as well as for twins. However, during the decade the mean maternal age increased only in the two hospitals with better socioeconomic levels. Gestational order decreased as socioeconomic levels increased, mainly for twins and triplets. The percentage of singletons with low birthweight and very low birthweight decreased as socioeconomic level increased. However, twins presented with an equal distribution in the four hospitals, indicating that better socioeconomic level did not affect the incidence of low birthweight and very low birthweight among twins. Fetal death rate decreased as socioeconomic level increased but twin/singleton fetal death ratio is three times greater in the hospital of higher socioeconomic level suggesting that even in ideal conditions of medical and hospital facilities, the mortality of twins continues to be much higher than that of singletons.

When twinning rates are studied, maternal age and parity should be considered. Data on parity are seldom available. We studied information about the mean parity, using the gross reproduction rate, the total fertility rate and the crude birth rate. These are strongly correlated with the mean parity. The crude birth rate is more readily available than the gross reproduction rate or the total fertility rate. Earlier studies have shown that it is difficult to model variations in the twinning rate with data for the macrolevel. In this study these findings are explained by theoretical analyses and illustrated by empirical data. Sweden, having the oldest continuous population statistics and high twinning rates, offers excellent possibilities for analyses of the twinning rate. We considered data for the counties of Gotland, Älvsborg and Stockholm and the city of Stockholm from 1749 to 1960 and for Sweden until 1996. For Älvsborg, the twinning rate was low for the whole period, showing no statistically significant decreasing trend. It is mainly about 11–14 per thousand, which is only 50–60% of the twinning rate in Gotland in the 18th century. In Gotland, in the county of Stockholm, in the city of Stockholm and in Sweden as a whole, the decreasing trends in the twinning rate were statistically significant. The decreasing twinning rates converge towards the low twinning rate of Älvsborg. After standardization of the twinning rate, the differences remained and the low rate in Älvsborg could not be explained by maternal age.

The relative importance of genetic influences (heritability) on five general textural quality characteristics of the human iris was assessed using sex and age limitation models. Colour photographs of irises were available from 100 monozygotic twin pairs, 99 dizygotic twin pairs, and 99 unrelated randomly paired age-matched German subjects. Comparative scales were constructed and two judges who were blind to zygosity independently rated five characteristic of the subjects' left iris. Inter-rater reliabilities were larger than .90 for all five scales. The heritabilities for the five iris characteristics ranged from .51–.90. No sex-specific genetic factors were found for the iris characteristics. Age-group differences in heritability were found for one of the five iris characteristics — “distinction of white dot rings”. Heritability was greater for the older cohort (90%) than the younger (73%). The iris characteristics that showed the next highest additive-genetic effect were “contractional furrows” (78%) and “frequency of crypts” in the main stroma leaf (66%).

Studies in Sweden and Australia have examined the relationship between number of CAG repeats in the androgen receptor gene and psychological traits — three Masculinity-Femininity (M-F) measures in Australia, and the Karolinska Scales of Personality (KSP) in Sweden. The present study derived M-F scales from the KSP items, and scales corresponding to several KSP scales from the items of the inventories used in Australia, to permit cross-validation of the Australian results in the Swedish sample, and vice versa. The derivation of scales was facilitated by the fact that items from both inventories had been used with a large twin sample in the US. Correlation of the derived scales with androgen receptor gene CAG-repeat scores for women in the Australian and Swedish samples failed to provide clear evidence of replication of either set of original correlations in the other sample, although there were a few hints of consistency. It was concluded that if the number of CAG repeats on this gene is related to psychological traits at all, the relationship is a weak one.

The relationship between regular smoking behavior and the smoking behavior of parents, siblings and friends was investigated using data from the Netherlands Twin Register. Cross-sectional analyses of data of 3906 twins showed significant associations between smoking behavior of the participant and smoking behavior of co-twin, additional brothers, parents of the same sex as the participant and friends. Those variables, together with age, explained 47% of the variance in smoking behavior. Longitudinal analyses of data from 2397 twins, who, in 1993, reported never to have smoked (regularly), showed that uptake of regular smoking two years later was predicted by having a smoking co-twin, smoking same-sex siblings, smoking mother and smoking friends. Males are, in contrast to females, at a later age still vulnerable to taking up regular smoking. The variables explained 21% of the variance. Sport participation, alcohol use, boredom susceptibility and neuroticism significantly added to the predictive value of this model. Including those additional factors increased the explained variance to 30%, and subsequently adding experimental smoking behavior further increased the explained variance to almost 50%. In summary, having smoking family members and friends, as well as lifestyle and personality factors are important predictors for the uptake of regular smoking. However, the experimental smoking behavior of the participant is equally important.

Combat-related posttraumatic stress disorder (PTSD) is highly comorbid with other mental disorders. However, the nature of the relationship between PTSD and other mental disorders remains unclear. A discordant high-risk twin design was used on data from a sub-sample of the male-male twin pair members of the Vietnam Era Twin Registry to examine whether patterns of comorbidity are consistent with a psychopathological response to combat exposure or reflect familial vulnerability to psychopathology. Mental disorders were assessed via the Mental Health Diagnostic Interview Schedule Version III — Revised. Discordant monozygotic within-pair comparisons revealed that PTSD probands had higher symptom counts and diagnostic prevalences of mood and anxiety disorders than their non-combat exposed co-twins. Monozygotic co-twins of PTSD probands had significantly more mood disorder symptoms than monozygotic co-twins of combat controls or dizygotic co-twins of veterans with PTSD. These findings suggest that a) major depression, generalized anxiety disorder and panic disorder are part of a post-combat response syndrome; b) a shared familial vulnerability also contributes to the association between PTSD and major depression, PTSD and dysthymia, and c) this shared vulnerability is mediated by genetic factors.

The authors define a trait “wellness” for good health in 6109 men in the National Academy of Sciences-National Research (NAS-NRC) twin panel aged 70 years and up surveyed by mail in the fall of 1998. Men who responded that they had not had a heart attack, coronary surgery, stroke, diabetes or prostate cancer in the survey questionnaire (Q8) met the broad definition of wellness. A more narrow definition included the absence of hypertension. Genetic analysis indicated that over 50% of the population variance for liability to either wellness definition was genetic. A subset of the NASNRC twins also participates in the National Heart, Lung and Blood Institute (NHLBI) twin study. NHLBI examinations and medical record review was done in 1986–1987 and 1995–1997 for 389 individuals who completed Q8. Excellent agreement (kappa > 0.8) was found between Q8 and outcome review for each condition comprising the wellness definition, ranging from 0.81 for coronary surgery to 0.88 for diabetes. Substantial agreement (kappa = 0.67) was found for hypertension. Kappa values for wellness were 0.82 for the broader definition and 0.74 if high blood pressure was included. Fraternal twin-pairs concordant for the wellness definitions are currently being recruited for linkage studies.

Most analyses of data collected from a classical twin study of monozygotic (MZ) and dizygotic (DZ) twins assume that zygosity has been diagnosed without error. However, large scale surveys frequently resort to questionnaire-based methods of diagnosis which classify twins as MZ or DZ with less than perfect accuracy. This article describes a mixture distribution approach to the analysis of twin data when zygosity is not perfectly diagnosed. Estimates of diagnostic accuracy are used to weight the likelihood of the data according to the probability that any given pair is either MZ or DZ. The performance of this method is compared to fully accurate diagnosis, and to the analysis of samples that include some misclassified pairs. Conventional analysis of samples containing misclassified pairs yields biased estimates of variance components, such that additive genetic variance (A) is underestimated while common environment (C) and specific environment (E) components are overestimated. The bias is non-trivial; for 10% misclassification, true values of Additive genetic: Common environment: Specific Environment variance components of .6: .2: .2 are estimated as .48: .29: .23, respectively. The mixture distribution yields unbiased estimates, while showing relatively little loss of statistical precision for misclassification rates of 15% or less. The method is shown to perform quite well even when no information on zygosity is available, and may be applied when pair-specific estimates of zygosity probabilities are available.

Two families with twins — one in which adult DZ twins continue to face the risk of frontotemporal dementia, and the other in which a mother copes with brittle bones in a newborn infant MZ pair — are featured. The nature and progress of the disorders are reviewed and personal perspectives from the twins and other family members are variously provided. Next, four current twin studies are briefly summarized. Topics include sex differences in social support systems, new reproductive technologies, comparative twin-singleton frequencies of placenta previa, and the nature and complications of heterotopic pregnancies. This article concludes with a brief sampling of human interest stories involving twins: a different looking DZ pair, newborn sextuplets, conjoined twins and opposite-sex twin athletes.