ABSTRACT
Twin studies typically indicate shared environmental influence for cognitive abilities, especially in early childhood. However, across studies, DZ twin correlations tend to be greater than non-twin sibling correlations, suggesting that twin estimates of shared environment are to some extent specific to twins. We tested this hypothesis in a sample of more than 1800 MZ and 1800 same-sex DZ pairs from the Twins Early Development Study (TEDS), a study of twins born in England and Wales in 1994 and 1995. For this analysis, we obtained comparable data from more than 130 same-sex younger siblings of the twins. Twins and their younger siblings were assessed for language, cognitive abilities and behavior problems by their parents at 2 and 3 years of age. For language and cognitive measures at both 2 and 3 years, but not for behavior problems, estimates of shared environment were more than twice as large for twins as compared to non-twin siblings. We conclude that about half of twin study estimates of shared environment for cognitive abilities in early childhood are specific to twins. Although many possibilities exist for explaining the special shared environment effect for twins, we suggest that cognitive-relevant experiences that are not shared by siblings are shared by twins because they are exactly the same age.
Subject(s)
Child Behavior Disorders/epidemiology , Cognition Disorders/epidemiology , Diseases in Twins/epidemiology , Environment , Language Disorders/epidemiology , Chi-Square Distribution , Child Behavior Disorders/genetics , Child, Preschool , Cognition Disorders/genetics , Diseases in Twins/genetics , England/epidemiology , Female , Humans , Language Disorders/genetics , Longitudinal Studies , Male , Siblings , Wales/epidemiologyABSTRACT
Normal language development was studied in 310 pairs of 4-year-old twins born in the United Kingdom in 1994. Twins were assessed individually in their homes on a diverse battery of language and nonverbal measures. Rotated factor analyses indicated the presence of a general Language factor (L) as well as a general Nonverbal (NV) factor. Moderate genetic influence was found for both L and NV abilities. Bivariate genetic analysis estimated a genetic correlation of .63 between L and NV abilities, implying that over half of the genetic influence on L overlaps with genetic influence on NV. These results suggest that at age 4, genetic influences on individual differences in language overlap substantially with genetic influences on individual differences in other cognitive abilities, although perhaps less so than later in development.
Subject(s)
Language Development , Twins/psychology , Child Language , Child, Preschool , Environment , Factor Analysis, Statistical , Female , Humans , Individuality , Language Tests/statistics & numerical data , Male , Multivariate Analysis , Nonverbal Communication/physiology , Phenotype , Sex Factors , Surveys and Questionnaires , Twins/genetics , United KingdomABSTRACT
Research on risk factors and pathogenesis of otitis media has emphasized the environment, but previous twin studies suggest a strong genetic component. In those studies, no attempt was made to differentiate the role of initial acute infection from the chronic airway blockage that frequently accompanies persistent effusion. The authors estimated genetic and environmental determination of both of these aspects of otitis media histories at three time points. A large and representative subset of a total population cohort of all twins born in England and Wales in 1994, was studied prospectively at ages 2, 3, and 4 years (monozygotic, n = 715; dizygotic, n = 658). For the total symptom score, the estimated heritabilities at ages 2, 3, and 4 years were 0.49, 0.66, and 0.71, respectively. All correlations for the total score found in monozygotic twins were about 0.9, but correlations were lower for dizygotic twins (p < 0.65). Shared environmental influences declined in importance over time (0.41, 0.22, and 0.16 at ages 2, 3, and 4 years, respectively). No gender differences were found in parameter estimates. Symptoms of acute infections showed lower heritability (0.57 on average) and higher shared environment (0.18) than did chronic airway blockage (0.72 heritability and 0.10 shared environment). The strong genetic component, which this study confirms, is not unitary but distributes differently across the phenotypic aspects of otitis media and probably across pathogenetic stage.
Subject(s)
Diseases in Twins/genetics , Otitis Media/genetics , Age Factors , Child, Preschool , Cohort Studies , England , Female , Genetic Predisposition to Disease/genetics , Humans , Longitudinal Studies , Male , Sex Factors , Twins, Dizygotic , Twins, Monozygotic , WalesABSTRACT
The Yoruba are an important ethnic group mainly occupying Southwestern Nigeria. Mainly for genetic reasons, this very large tribe happens to present the highest dizygotic twinning rate in the world (4.4 % of all maternities). The high perinatal mortality rate associated with such pregnancies has contributed to the integration of a special twin belief system within the African traditional religion of this tribe. The latter is based on the concept of a supreme deity called Olodumare or Olorun, assisted by a series of secondary gods (Orisha) while Yoruba religion also involves immortality and reincarnation of the soul based on the animistic cult of ancestors. Twins are therefore given special names and believed to detain special preternatural powers. In keeping with their refined artistic tradition, the Yoruba have produced numerous wooden statuettes called Ibejis that represent the souls of deceased newborn twins and are involved in elaborate rituals. Among Yoruba traditional beliefs and lore some twin-related themes are represented which are also found in other parts of the world. Basic features of the original Yoruba beliefs have found their way into the religious traditions of descendants of African slaves imported in the West Indies and in South America.
