Evidence continues to accumulate that Weinberg's Differential Rule is a useful approximation. This is in spite of the fact that one of the premises he used in its derivation is almost certainly, to some extent, false. An explanation is offered here.

Twin research exploring genetic and environmental influences on athletic interests and talents is reviewed. Illustrative examples of twin athletes representing a variety of sports activities are presented. This is followed by an overview of twin studies offering critical insights into the onset and progress of leukemia. In the last section, timely events involving twins and parents of twins will be described — each case provides a new look at an old question.

The present study examined the possible effects of chorionicity of twins on variation of prosocial behavior in the classical twin design. Mothers of 56 pairs of monochorionic monozygotic (MCMZ) twins, 34 pairs of dichorionic monozygotic (DCMZ) twins, and 316 pairs of dizygotic (DZ) twins rated their children's prosocial behavior. The MCMZ correlation for prosocial behavior was similar to the DCMZ correlation (.63 vs. 61), but both correlations were higher than the DZ correlation (.33). Models incorporating the chorion effects were fit to the data. Genetic, shared and nonshared environmental, and chorion effects in the full model were, respectively, 53% (95% CI: 1–70%), 7% (95% CI: 0–37%) , 40% (95% CI: 29–54%), and 0% (95% CI: 0–27%), with the effects of chorion and shared environment being nonsignificant. These findings indicate that genetic and environmental factors in prosocial behavior estimated from twin studies are not significantly influenced by the chorion type of MZ twins.

Twins compared with singletons and monozygous (MZ) compared with dizygous (DZ) twins are at increased risk of fetal and infant death, cerebral palsy and many congenital anomalies. The aim of this study is to investigate whether zygosity is a risk factor for the sudden infant death syndrome (SIDS). Birth registration data and draft infant death certificates for all multiple births in England and Wales 1993 to 2003 were provided by the Office for National Statistics. As a partial proxy for zygosity, same-sex was compared with opposite-sex twins for birthweight-specific mortality and mortality attributed to SIDS. Data on singleton infants were obtained by subtraction of multiple births from routinely published population births and infant deaths. SIDS mortality among low birthweight infants was significantly less in twins than singletons. The twin-singleton relative risk was reversed in infants of normal birthweight. Among infants of normal birthweight, neonatal SIDS was significantly more common in same- compared with opposite-sex pairs. Among infants of low birthweight, postneonatal SIDS was significantly more common in same- compared with opposite-sex pairs. The difference in birthweight distribution of same- compared with opposite-sex twins for neonatal SIDS suggests that zygosity is a risk factor for SIDS. As congenital cerebral anomalies are a feature of many monozygous twin conceptions, a detailed macro- and microscopical examination of the brain in twin SIDS may indicate an otherwise unrecognised pathology.

Human growth is a complex and poorly understood process. We studied the effect of genetic and environmental factors on height and body mass index (BMI, kg/m2) based on maternal reports at 3, 4, 5, 7, 10 and 12 years of age in a large longitudinal cohort of Dutch twins (7755 complete twin pairs at age 3). Several multivariate variance component models for twins were fitted to the data using the Mx statistical package. The first-born twin was taller until age 10 and heavier until age 12 than the second-born co-twin. Heritability estimates were high for height (a2 = .58–.91) and BMI (a2 = .31–.82), but common and unshared environmental factors were also important. The phenotypic correlations across the ages for height and BMI were mainly explained by correlated additive genetic factors (ra = .77–.96 for height and .43–.92 for BMI), but common (rc = .40–.84 and .09–.78, respectively) and specific environmental correlations (re = .50–.81 and .42–.80, respectively) were also significant. Additive genetic factors decreased with increasing age difference for both height and BMI. However, the full Cholesky model, which does not make any assumptions regarding the underlying genetic structure, had the best fit. High genetic correlations across the ages, especially for height, may help further molecular genetic studies of human growth. Environmental factors affecting height and BMI during growth period are also important, and further studies are needed to identify these factors and test whether they interact with genetic factors.

The Wisconsin Twin Panel utilizes the resources of state birth records to study the etiology and developmental course of early emotions, temperament, childhood anxiety and impulsivity, the autism spectrum, and related psychobiological and behavioral phenotypes. The panel currently supports 5 active research studies which involve twins from birth to early adolescence. A range of research methods are employed, including questionnaires and structured interviews with caregivers, home and laboratory-based behavioral batteries, observer ratings, child self-report, psychophysiology, neuroendocrine measures, birth records, genotyping, and cognitive testing. The panel is in the early stages of generating longitudinal findings.

Plasma lipids such as high-density lipoprotein (HDL), low-density lipoprotein (LDL), total cholesterol and triglyceride levels contribute to variation in the risk of cardiovascular disease. The early stages of atherosclerosis in childhood have also been associated with changes in triglycerides, LDL and HDL. Heritability estimates for lipids and lipoproteins for adolescents are in the range .71 to .82, but little is known about changes of genetic and environmental influences over time in adolescence. We have investigated the contribution of genetic and environmental influences to variation in lipids in adolescent twins and their nontwin siblings using longitudinal twin and family data. Plasma HDL and LDL cholesterol, total cholesterol and triglycerides data from 965 twin pairs at 12, 14 and 16 years of age and their siblings have been analyzed. Longitudinal genetic models that included effects of age, sex and their interaction were fitted to assess whether the same or different genes influence each trait at different ages. Results suggested that more than one genetic factor influences HDL, LDL, total cholesterol and triglycerides over time at ages 12, 14 and 16 years. There was no evidence of shared environmental effects except for HDL and little evidence of long-term nonshared environmental effects was found. Our study suggested that there are developmental changes in the genes affecting plasma lipid concentrations across adolescence.

Our aim was to evaluate the risk of death among healthy infants due to maltreatment, using national linked vital statistics data. The study population included all nonanomalous, full-term (≥ 37 weeks), non-low birthweight (> 2500 grams) infants born between 1995 and 2000 and their linked death certificates: 18,673,439 singleton pregnancies (36,864 deaths) and 77,800 twin pregnancies (356 pregnancies with a death). The underlying cause of death was characterized as due to maltreatment, sudden infant death syndrome (SIDS), and other causes, based on ICD-9 and ICD-10 codes, and modeled by maternal age using multinomial logistic regression; mothers aged 25 to 29 were the reference group. The highest risk for infant mortality was among the youngest mothers for maltreatment (AOR 2.45 and 1.95 for singleton mothers < 20 and aged 20 to 24, respectively; AOR 4.34 and 2.25 for twin mothers < 20 and aged 20 to 24, respectively). The risk of death overall and for each category was modeled by maternal age < 20, aged 20 to 24, and ≥ 25, with and without the father's age present on the birth certificate, with mothers ≥ 25 and father's age present as the reference group. All risks of death were significantly increased for mothers younger than age 25, with the highest risks among the youngest mothers and missing father's age. The pattern for twins was similar, with elevated risks among younger mothers with or without father's age present on the birth certificate. These results add to the body of knowledge regarding risk factors for infant mortality among healthy singletons and twins.

We investigated whether women show larger heart rate variability (HRV) than men after controlling for a large number of health-related covariates, using two indices of HRV, namely respiratory sinus arrhythmia (RSA) and approximate entropy (ApEn). In a twin design, the heritability of both indices was examined. The covariation between RSA and ApEn, a measure of heart rate dynamics derived from nonlinear dynamical systems theory, was decomposed into genetic and environmental components. Subjects were 196 male and 210 female middle-aged twins. Females showed larger HRV than men before (ApEn: p < .001; RSA: p = .052) and after adjustment for covariates (ApEn: p < .001; RSA: p = .015). This sex difference was confirmed by significant intrapair differences in the opposite-sex twin pairs for both ApEn (p < .001) and RSA (p = .03). In addition to sex, only heart period and age (both p < .001) were found to be independent predictors of ApEn, whereas RSA was also influenced by respiration rate and smoking (both p < .001). Age explained 16% and 6% of the variance in RSA and ApEn, respectively. Oral contraceptive use and menopausal status had no effect on HRV. Genetic model fitting yielded moderate heritability estimates for RSA (30%) and ApEn (40%) for both males and females. The correlation between RSA and ApEn (r = .60) could be attributed to genetic factors (48%), environmental factors (36%) and age (16%). The present study found support for a gender difference in HRV with women having greater HRV than men even after controlling for a large number of potential confounders. Indices of heart rate dynamics derived from nonlinear dynamical systems theory are moderately heritable and may be more sensitive than traditional indices of HRV to reveal subtle sex differences with important implications for health and disease.

The aim was to study the effect of twin gestations in a uterus with 2 bodies on the probability of breech presentation at delivery. The hypothesis was that the probability of breech presentation was not higher than 50%. A review was undertaken of MEDLINE (1966–2004) and of the article reference list for statistical analysis of presentation at delivery among twins in a normal uterus, singleton gestations in a uterus with 2 bodies, and case studies of twins in a uterus with 2 bodies. There are 10 studies of twin gestations in a normal uterus (Twin A 3036 cases, breech presentation 22.36%; Twin B 2758 cases, breech presentation 36.87%), 2 studies of singleton gestations in a uterus with 2 bodies (297 cases, breech presentation 42.09%), and 57 case report studies of twin gestations in a uterus with 2 bodies (Twin A 56 cases, breech presentation 14.29%; Twin B 54 cases, breech presentation 18.52%). The odds ratio and chi-square test for differences in probabilities show a significantly lower incidence of breech presentation for twins in a uterus with 2 bodies compared with twins in a normal uterus (Twin A, odds ratio = 0.58; χ2 = 2.08, p > .05, Twin B, odds ratio = 0.39, χ2 = 7.67, p < .05), and singleton gestations in a uterus with 2 bodies (Twin A, odds ratio = 0.23, χ2 = 15.51, p < .05; Twin B, odds ratio = 0.31, χ2 = 10.72, p < .05). Twin gestations in a uterus with 2 bodies decrease the probability of breech presentation.

In a previously published article (Resta et al., 2006) on Robert's syndrome in prenatal diagnosis, a case of a 36-year-old woman and her 36-year-old, nonconsanguineous husband were presented. Our findings suggest the existence of nonsense mediated decay (NMD) variability which could account for the varying severity reported in carriers of identical mutations. Furthermore, fetal cells were used to evaluate the influence of premature centromere separation (PCS) on the sister chromatid exchange (SCE) and micronucleus (MN) frequency. Given the similar variation observed in the SCE frequencies, dependent on tissue/cell type (amniotic fluid sample, chorionic villus sampling) and duration of in vitro cultures (48 hours or 72 hours), the idea was that this new piece of information could be interesting. It seems that the SCE frequency increased proportionally to the cell cycle increasing (1° < 2° < 3° … n). Obviously, our observations are too scarce to draw conclusions, but further investigation could be useful to corroborate or dispute these results, considering that the two techniques, (MN and SCE), are simple to perform and do not require expensive laboratory equipment.

In recent years, there has been growing appreciation for the complexity of gender identity. Focusing on monozygotic (MZ) twins discordant for transsexualism can offer clues to events that may trigger this behavioral difference, offering new information about critical underlying factors. An update of twin research in this area is provided, together with a preview of a compelling new film, ‘Red Without Blue.’ Next, twin study findings on the topics of conjoined twinning, angiographic lesions and embryo transfer are provided. This is followed by a survey of newsworthy twins and twin-related events.

Adoption agencies are expending greater efforts toward placing adoptive twins and siblings together in the same home. However, new sources of reared apart twins continue to be identified. Some of these sources are described, with suggestions for how individual cases can be used more fruitfully by interested researchers. Next, recent findings on the fitness of female twins from male–female pairs and the heritability of olfactory thresholds are summarized. Finally, new thoughts about some artificially conceived infants and artificial reproductive methods are considered. A generous donation to the Twins Days Festival, in Twinsburg, Ohio is also described.

While evidence supporting for nonadditive genetic influences on personality traits in Caucasian populations has been growing in recent years, twin studies that explored the existence of genetic nonadditivity in personality variation in Asian populations are still lacking. Seven hundred and sixty-five pairs of adolescent and young adult twins registered with the South Korean Twin Registry completed the 7 scales of the Eysenck Personality Scales through a mail survey. Maximum likelihood twin correlations were computed and model-fitting analyses were conducted. Monozygotic twin correlations were consistently higher than twice the dizygotic twin correlations for all 7 scales, suggesting pervasive influences of nonadditive genetic effects on personality traits in the South Korean population. Model-fitting analyses indicated that genetic nonadditivity is particularly important for the variation of Impulsivity, Venturesomeness, Empathy, Lie, and Psychoticism. According to the best fitting models, nonadditive genetic effects ranged from 34 to 49% for these scales. For Neuroticism and Extraversion, models that included an additive genetic component fit better than those including a nonadditive genetic variance component.

Considerable evidence from twin and adoption studies indicates that both genetic and shared environmental factors play a substantial role in the liability to antisocial behavior. Although twin and adoption designs can resolve genetic and environmental influences, they do not provide information about assortative mating, parent–offspring transmission, or the contribution of these factors to trait variation. We examined the role of genetic and environmental factors for conduct disorder (CD) using a twin–parent design. This design allows the simultaneous estimation of additive genetic, shared and individual-specific environmental effects, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission. A retrospective measure of CD was obtained from twins and their parents or guardians in the Virginia Twin Study of Adolescent Behavior Development and its Young Adult Follow up sample. Both genetic and environmental factors play a significant role in the liability to CD. Major influences on individual differences appeared to be additive genetic (38%–40%) and unique environmental (39%–42%) effects, with smaller contributions from the shared environment (18%–23%), assortative mating (~2%), cultural transmission (~2%) and resulting genotype-environment covariance. This study showed significant heritability, which is slightly increased by assortative mating, and significant effects of primarily nonparental shared environment on CD.

Some legal and social complications that arise when identical twins have children are examined. The specific case presented concerns assignment of paternity. This section is followed by a review of recent studies of multiple birth pregnancy risk factors, namely mother to infant HIV transmission and congenital hypothyroidism. Honors given to recently recognized twin researchers are noted, as is the birth of twins to an older mother in the United States.

This study was performed using population-based data to analyze whether motor development in early life is different between singletons and twins in Japan. For better comparison and investigation, we divided the 2 groups into a group with a birthweight of 2.5 kg or greater (subgroup A) and a group with a birthweight of less than 2.5 kg (subgroup B), respectively. We analyzed the database of medical check-ups for children aged 3 years between April 2001 and July 2004. They received medical checkups at 4 months, 1.5, and 3 years of age. Children who were suspected of having neurological abnormality or disability were referred to specialists and excluded from the database. The data of 14,132 children were analyzed. Among these, 13,040 (92.3%) children were singletons in subgroup A, 75 (0.5%) were twins in subgroup A, 866 (6.1%) were singletons in subgroup B and 151 (1.1%) were twins in subgroup B. The mean age at achieving milestones was slower in twins of subgroup A for each developmental outcome than singletons of subgroup A, and the difference between twins and singletons was significant after adjustment for rolling over. On the contrary, after adjusting for a confounding factor (gestational age), singletons of subgroup B attained motor development facilitating walking independently slower than twins of subgroup B. There were different tendencies in the results regarding the motor development of subjects of subgroup A and that of subjects of subgroup B.