Hereditary influences account for a substantial proportion of the variance in many cognitive abilities. However, there is increasing recognition that the relative importance of genetic and environmental influences may vary across different socioeconomic levels. The overall goal of the present study was to examine whether parental education has a moderating effect on genetic and environmental in...

The genetic contribution to dizygotic (DZ) twinning was investigated using 6,596 twin pairs from the Australian Twin Registry who provided information on other twins in their families. Responses were classified by the zygosity (DZ; monozygotic [MZ]) of the proband twins and by the relationship and zygosity of related twins. MZ probands and MZ twins reported by DZ probands were used as controls ...

OBJECTIVES We examined whether and when differences in body size disappear over time and whether twins attain normal final height and body mass index (BMI). STUDY DESIGN Height, weight, and BMI data of twins at ages 1, 4, and 18 years were compared with data from their nontwin siblings. Second, twin and sibling data were compared with population standards. In addition to height, weight, and B...

Being an identical twin does not necessarily mean having identical perceptions of family functioning, nor of the twin relationship. Using the co-twin control design, the aim of this study was to explore perceptions of family dynamics and the twin relationship in monozygotic (MZ) twins discordant and concordant for Developmental Coordination Disorder (DCD). It was hypothesized that, as has been ...

OBJECTIVE Although previous studies have suggested that sexual orientation is influenced by familial factors, which may be partly genetic, these studies have relied on unrepresentative and potentially biased samples. The authors attempted to estimate the role of genetic and environmental factors in the determination of sexual orientation in a more representative sample. METHOD Sexual orientat...

Twin research has offered evidence that monozygotic (MZ) twins are more socially close than dizygotic (DZ) twins, but has not paid much attention to the way twins compare themselves with their co-twin. The few studies in this area suggest that 'horizontal comparisons' (social comparison motivated by solidarity or communion with others) matter more for MZ twins than for DZ twins, at least when t...

Conjoined twins are twins whose bodies are anatomically joined in utero. The degree to which the twins are attached can range from simple, involving skin and cartilage, to complex, including fusion of the skull(s), brain(s), or other vital organs. There are more than a dozen classifications of conjoined twins [3] but what they all tend to have in common is the sharing of the chorion [4], placen...