How intelligence and education contribute to substance use: Hints from the Minnesota Twin family study

In old and even middle age, there are associations between physical health and both intelligence and education. This may occur because intelligence and/or education exert effects on lifestyle choices that, in turn, affect later health. Substance use is one aspect of lifestyle choice in young adulthood that could play such a role. The effects of intelligence and/or education on substance use could be direct and environmental, or indirect due to the presence of confounding genetic and shared family influences. We used the Minnesota Twin Family Study to distinguish these effects in males and females at age 24. In contrast to prevailing expectations, there were moderately negative direct nonshared environmental effects of both IQ and education on both smoking and drinking in both males and females. That is, controlling for family background effects in the form of both genetic and shared environmental influences, both higher IQ and greater education were associated with greater alcohol and nicotine use. These effects were accounted for by alcohol and nicotine use at age 17. Our results suggest that genetic and family-culture variables confound the associations between intelligence and education and substance use in young adults, rendering them indirect. Further research is needed to understand the roles of IQ and education in alcohol and nicotine use and their relative impacts on physical health throughout the lifespan.     

Socioeconomic status and school grades: Placing their association in broader context in a sample of biological and adoptive families

SES has long interested researchers investigating school achievement. Its effects are often addressed by studying predictors of achievement in economically disadvantaged samples living primarily in biological families, confounding genetic and environmental influences. Little is known about SES's purely environmental effects. We measured them in 617 adoptive and biological families, adjusting for sample restriction of SES range. Controlling for gender, parenting, parental expectations for educational attainment (PEEA), IQ, engagement in school, and genetic and shared environmental influences on sibling pairs, SES still made a small but significant nonshared environmental contribution to school grades. IQ, PEEA, and SES had collinear associations with school grades, as did engagement and parenting. The associations of IQ and engagement with school grades were largely independent of each other. The link between PEEA and IQ was stronger in adoptive than biological offspring. We discuss the implications of these findings.     

Genetic influences on the overlap between low IQ and antisocial behavior in young children

The well-documented relation between the phenotypes of low IQ and childhood antisocial behavior could be explained by either common genetic influences or environmental influences. These competing explanations were examined through use of the Environmental Risk Longitudinal Twin Study 1994-1995 cohort (Moffitt & the E-Risk Study Team, 2002) of 1,116 twin pairs and their families. Children's IQ was assessed via individual testing at age 5 years. Mothers and teachers reported on children's antisocial behavior at ages 5 and 7 years. Low IQ was related to antisocial behavior at age 5 years and predicted relatively higher antisocial behavior scores at age 7 years when antisocial behavior at age 5 years was controlled. This association was significantly stronger among boys than among girls. Genetic influences common to both phenotypes explained 100% of the low IQ-antisocial behavior relation in boys. Findings suggest that specific candidate genes and neurobiological processes should be tested in relation to both phenotypes.     

Genetic and environmental influences on individual differences in printed word recognition

The genetic and environmental etiologies of individual differences in printed word recognition and related skills were explored in 440 identical and fraternal twin pairs between 8 and 18 years of age. A theoretically driven measurement model identified five latent variables: IQ, phoneme awareness, word recognition, phonological decoding, and orthographic coding. Cholesky decomposition models on these five latent constructs revealed the existence of both common and independent genetic effects, as well as non-shared environmental influences. There was evidence for moderate genetic influences common between IQ, phoneme awareness, and word-reading skills, and for stronger IQ-independent genetic influences that were common between phoneme awareness and word-reading skills, particularly phonological decoding. Phonological and orthographic coding skills in word recognition had both significant common and significant independent genetic influences, with implications for "dual-route" and "connectionist" reading models, subtypes of reading disabilities, and the remediation of reading disabilities.     

Herrnstein''s syllogism: genetic and shared environmental influences on IQ, education, and income

Genotypes may influence the phenotypic associations among IQ, education, and income. To investigate this hypothesis, we believe that the appropriate methodology requires estimation of genetic and environmental influences using data able to separate these influences. The National Longitudinal Survey of Youth (NLSY) is a nationally representative sample that contains genetically-informative full- and half-siblings (28–35 years old in 1992; Ns=1943 full-siblings, 129 half-siblings). A biometric genetic model was fit that estimated the shared environmental and genetic variance components of IQ, years of education, and hourly income. The total heritabilities were 0.64 for IQ, 0.68 for education, and 0.42 for income. Heritabilities due to a common genetic factor were 0.35 for IQ, 0.52 for education, and 0.12 for income. Environmental influences due to a common shared environmental factor were 0.23 for IQ, 0.18 for education, and 0.08 for income. The model predicted a correlation of 0.63 between IQ and education and 0.34 between IQ and income. Sixty-eight percent of the former and 59% of the latter was genetically mediated; the remainder was mediated by common shared environment. These findings suggest that social inequality in the United States has its origin in both genetically-based traits and in different environmental backgrounds.     

The role of task persistence in young adolescence for successful educational and occupational attainment in middle adulthood

The main purpose of the present study was to investigate the importance of task persistence in young adolescence for successful educational and occupational attainment in middle adulthood. Data from age 13 (N = 1,092) and adult age (age 43 for women, N = 569 and age 47 for men, N = 393) were taken from the Swedish longitudinal research program entitled "Individual Development and Adaptation." In line with previous research, task persistence was found to be related to changes in grades between age 13 and age 16, over and above other childhood factors. Task persistence at age 13 was also a significant predictor of both income and occupational level in middle adulthood for the men, controlling for a number of childhood factors (including intelligence), and even when educational attainment in adulthood was taken into account. Finally, task persistence was related to educational attainment in adulthood. The authors suggest that task persistence is a second fundamental factor besides general mental ability, influencing attainment within the area of working life and education. The influence of task persistence is discussed in form of personality-environment selection mechanisms.     

Models of Educational Career With and Without IQ Measurements

Path models of educational careers do not usually contain information about phenotypical intelligence levels. This may lead to an overestimation of direct environmental effects such as the educational level of the parents on the education attained by the children. This phenomenon was investigated on the basis of data describing educational level, occupational status, and intelligence measurements of 2,847 father-son pairs. The IQ was determined during testing for military service by means of a shortened version of Raven's Progressive Matrices Test. The results were twofold. When the son's IQ was included in the model describing his educational career, environmental effects due to the influence of the father were reduced. Adding the father's IQ to the model, however, did not affect the path coefficients of the variables that were predicted to be relevant for the son's education.     

Education and Social Trust: Testing a Causal Hypothesis Using the Discordant Twin Design

One of the clearest results in previous studies on social trust is the robust positive relationship with educational attainment. The most common interpretation is that education has a causal effect on social trust. The theoretical argument and empirical results in this article suggest a different interpretation. We argue that common preadult factors such as cognitive abilities and personality traits rooted in genes and early‐life family environment may confound the relationship between educational attainment and social trust. We provide new evidence on this question by utilizing the quasi‐experiment of twinning. By looking at the relationship between education and social trust within monozygotic (MZ) twin pairs, we are able to avoid potential confounders rooted in genetic factors and common environmental influences because the monozygotic twins share both. The results suggest that when controlling for such familial factors the estimated effects of education on social trust are close to zero and far from reaching statistical significance. Further analyses show that the relationship between education and social trust largely is driven by common genetic factors.     

IQ in childhood and the metabolic syndrome in middle age: Extended follow-up of the 1946 British Birth Cohort Study

IQ in early adulthood has been inversely associated with risk of the metabolic syndrome in midlife. We tested this association in the British 1946 birth cohort, which assessed IQ at age eight years and ascertained the metabolic syndrome at age 53 years based on modified (non-fasting blood) ATPIII criteria. Childhood IQ was inversely associated with risk of the metabolic syndrome, but this association was almost entirely mediated by educational attainment and achieved occupational social class. This may be consistent with a pattern where childhood IQ is strongly associated with outcomes that reflect neurological disorder, such as the degenerative dementias, but less so with common chronic physical diseases of ageing.     

Childhood intelligence and adult health: The mediating roles of education and socioeconomic status

The longitudinal relation between childhood intelligence and various health outcomes in adulthood is now well-established. One mediational model that accounts for this relation proposes that intelligence has cumulative indirect effects on adult health via subsequent educational attainment and adult socioeconomic status (SES). The aim of the present study was to examine whether and the extent to which educational attainment and SES mediate the impact of childhood intelligence on three dimensions of adult health in Luxembourg, a country with high-quality universal public health care. We used data from 745 participants in the Luxembourgish MAGRIP study. At the age of 12, participants completed a comprehensive intelligence test. At the age of 52, they reported their educational careers, SES, and functional, subjective, and physical health status. Using structural equation modeling, we investigated the direct and indirect effects (via educational attainment and adult SES) of childhood intelligence on adult health. We found that higher childhood intelligence predicted better functional, subjective, and physical health in adulthood. These effects were entirely mediated via educational attainment and SES. The mediational processes differed depending on the health dimension under investigation: Whereas SES was crucial in mediating the effect of intelligence on functional and subjective health, educational attainment was crucial in mediating the effect on physical health. These findings held up when considering adult intelligence and were similar for women and men. Our results suggest that even excellent public health care cannot fully offset the cumulative effects of childhood intelligence on adult health. Further studies are needed to investigate the relative importance of different mediators in the intelligence–health relation while including a broader set of objective health measures.     

Determinants of educational achievement at 16+: Intelligence,personality, home background and school

The classical path model linking home background with IQ and school type to educational attainment is extended by the inclusion of the EPQ and new questionnaires measuring Status Aspiration and Work Ethic. In a study of approx. 700 adolescents it was shown that Psychoticism and Status Aspiration are significant variables in the prediction of educational achievement. The most important determinant of educational attainment was found to be intelligence.     

Intergenerational class mobility in Britain: A comparative look across three generations in the Lothian Birth Cohort 1936

The Lothian Birth Cohort 1936 sample is in a uniquely good position to provide relevant data on social class mobility patterns over most of the last century. These participants, with known ultimate social class attainment, took a validated mental test in the Scottish Mental Survey of 1947 and were followed up at approximately age 70. Then, besides their own age 11 mental ability data and educational and occupational attainment, they reported educational and occupational data on their parents and offspring. We constructed structural equation models of participant and offspring social class attainment and odds of social class movement, and compared results with those from the similarly recruited and assessed Lothian Birth Cohort 1921. In the 1936 cohort, relative transmission of social class from one generation to the next decreased, while relative transmission of educational attainment increased. In addition, effects of childhood IQ and educational attainment on social class decreased, apparently due to relative decrease in overall upward mobility and absolute increase in overall education level.     

Intellectual similarity of virtual twin pairs: Developmental trends

Virtual twins (VTs) are same-age unrelated siblings reared together from infancy who replicate twinship, but without the genetic relatedness. A 2005 report from the ongoing Fullerton Virtual Twin Study found an IQ intraclass correlation of .26 (p < .01, n = 113 pairs) and a within-pair difference of 13.22 IQ points. The average age of VTs in that study was 8.10 years (SD = 8.56, range: 4.01-54.84 years). An opportunity to retest members of 43 VT pairs, 1.70-8.96 years after their time 1 assessment, allowed additional analyses of genetic and environmental influences underlying general intellectual development. A decrease in the VT IQ correlation and an increase in the within-pair difference were indicated, consistent with increasing genetic and/or non-shared environmental influences and decreasing shared environmental influence on general intellectual development throughout childhood.     

Personality related traits as predictors of music practice: Underlying environmental and genetic influences

Little is known about reasons for individual differences in practice behavior – why do some individuals practice more than others? Here we explore personality related traits such as openness, motivation and flow proneness as well as IQ as potential predictors of music practice. Using a large Swedish twin cohort of more than 10,500 individuals we also estimated genetic and environmental influences underlying such associations. Significant associations with music practice were found for IQ, intrinsic motivation, music flow, and openness. With all predictors in the same model (including sex and age) we could explain about 25% of variance in music practice. However, IQ and intrinsic motivation became non-significant in the full model, with music specific flow being the strongest predictor of music practice. Multivariate genetic modeling with the two remaining significant predictors (openness and music flow) and music practice suggested that the associations between the variables were largely due to shared genetic influences with some additional non-shared environmental influences. Our findings suggest that common genes may influence both music practicing behavior and traits related to artistic interests and musical enjoyment (flow).     

Investigating cognitive transfer within the framework of music practice: genetic pleiotropy rather than causality

The idea of far transfer effects in the cognitive sciences has received much attention in recent years. One domain where far transfer effects have frequently been reported is music education, with the prevailing idea that music practice entails an increase in cognitive ability (IQ). While cross‐sectional studies consistently find significant associations between music practice and IQ, randomized controlled trials, however, report mixed results. An alternative to the hypothesis of cognitive transfer effects is that some underlying factors, such as shared genes, influence practice behaviour and IQ causing associations on the phenotypic level. Here we explored the hypothesis of far transfer within the framework of music practice. A co‐twin control design combined with classical twin‐modelling based on a sample of more than 10,500 twins was used to explore causal associations between music practice and IQ as well as underlying genetic and environmental influences. As expected, phenotypic associations were moderate (r = 0.11 and r = 0.10 for males and females, respectively). However, the relationship disappeared when controlling for genetic and shared environmental influences using the co‐twin control method, indicating that a highly practiced twin did not have higher IQ than the untrained co‐twin. In line with that finding, the relationship between practice and IQ was mostly due to shared genetic influences. Findings strongly suggest that associations between music practice and IQ in the general population are non‐causal in nature. The implications of the present findings for research on plasticity, modularity, and transfer are discussed.     

Intelligence,social class of origin,childhood behavior disturbance and education as predictors of status attainment in midlife in men: The Aberdeen Children of the 1950s study

In a birth cohort of 6281 men from Aberdeen, Scotland, social class of origin, childhood intelligence, childhood behavior disturbance and education were examined as predictors of status attainment in midlife (46 to 51 years). Social class of origin, intelligence and behavior disturbance were conceptualized as correlated predictors, whose effects were hypothesized to be partially mediated by educational qualifications. A structural equation model using Full Information Maximum Likelihood estimation confirmed that education had the strongest direct effect on status attainment at midlife. Furthermore, education partially mediated the effects of social class of origin and childhood intelligence, and fully mediated the effects of behavior disturbance on status attainment. Social class of origin, childhood intelligence and behavior disturbance were strongly inter-correlated. After controlling for these associations, educational and social status attainments were influenced to a considerably greater extent by childhood intelligence than by social class of origin.     

Genetic and environmental influences on academic achievement trajectories during adolescence

Most studies have considered the effects of particular characteristics on academic achievement individually, which means that little is known about how they function together. Using the population-based Minnesota Twin Family Study, the authors investigated the effects of child academic engagement (interest, involvement, effort), IQ, depression, externalizing behavior, and family environmental risk on academic achievement (reported school grades) from ages 11 through 17. Hierarchical linear growth curve modeling showed main effects on initial reported Grades for all variables, and IQ mitigated the deleterious effects of family risk and externalizing. Only engagement affected change in Grades through adolescence. Influences on initial Grades were strongly genetically influenced, associated primarily with IQ, engagement, and externalizing behavior. Shared environmental influences on initial Grades linked engagement, IQ, and family risk. Genetic influences on change in Grades were substantial, but they were not associated with the academic, family risk, and mental health covarying factors. These results indicate that age 11 achievement and change in achievement through adolescence show systematic patterns and document the existence of individual differences in the commonly shared developmental experience of adapting to the school environment.     

Parental interest in children's education,children's self‐esteem and locus of control,and later educational attainment: Twenty‐six year follow‐up of the 1970 British Birth Cohort

Background. Few studies have investigated if mother's interest and father's interest in child's education are linked to educational attainment via their impact on child's self‐esteem and locus of control. Aims. (1) To investigate (after controlling for known confounding factors) the long‐term effect of mother's and father's interest in child's education at age 10 and child's locus of control and self‐esteem at age 10 in educational attainment at age 26; and (2) to explore if mother's interest and father's interest in child's education are linked to child's educational attainment via their effect in increasing child's self‐esteem and internal locus of control. Sample. The study used longitudinal data from sweeps of the 1970 British Cohort Study (BCS70). The initial sample was those 1,737 men and 2,033 women with valid data on age 10 self‐esteem, locus of control, father's interest, mother's interest, and age 26 educational attainment. Of these, 1,326 men and 1,578 women were included in the final analysis. Method. The birth to age 10 factors that were controlled for were birth weight, parental social class, socio‐economic disadvantage, emotional/behavioural problems, cognitive ability, and mother's educational attainment. Results. At the multivariate level, internal locus of control and mother's interest (but not self‐esteem) were significantly related to educational attainment in both men and women. Father's interest was a significant predictor of educational attainment only in women. Parent's interest was not linked to educational attainment via its impact on child's self‐esteem or locus of control. Self‐esteem predicted educational attainment in both genders by increasing internal locus of control, and fathers' interest predicted educational attainment in men by increasing mother's involvement. Conclusion. Although mothers' and fathers' interest in their children's education were not linked to educational attainment via their impact on children's self‐esteem or locus of control, they were significant predictors of educational attainment especially in daughters.     

Educational attainment, intelligence, interstate migration, and suicide rates in the United States: rejoinder to Abel and Kruger (2005)

Abel and Kruger (2005) reported a negative association of educational attainment and suicide rate across the United States. Given strong links between intelligence and educational attainment, this appears to be inconsistent with positive associations of intelligence and suicide rate reported in several other geographical studies. However, the apparent inconsistencies may reflect interstate migration within the U.S. adult population. Testing this hypothesis showed that interstate migration was more strongly related to state suicide rates than educational attainment. Thus, the different results obtained by Abel and Kruger could reflect migration effects, which for the U.S. might render adult-age indicators of educational attainment inappropriate to reflect regional variation in intelligence by place of birth.     

The genetic and environmental architecture to the stability of IQ: Results from two independent samples of kinship pairs

Existing research has revealed that IQ remains relatively stable over the life course, though questions remain about how stable IQ is and whether the stability of IQ varies across different developmental periods of the life course. Despite this stability, there are also questions surrounding the factors that might explain the stability of IQ. Against this backdrop, we conduct bivariate genetic models to estimate genetic, shared environmental, and nonshared environmental influences on the stability of IQ. To do so, we analyze kinship pairs drawn from two separate longitudinal samples: The National Collaborative Perinatal Project (CPP) and the National Longitudinal Study of Adolescent Health (Add Health). Across both samples, IQ was found to be relatively stable. Moreover, the genetic analyses revealed that between 66% and 83% of the stability in IQ was due to genetic factors and between 43% and 69% of the change in IQ was due to genetic factors. The remainder of the stability and change in IQ was the result of a combination of shared and nonshared environmental influences. Importantly, some substantive race differences emerged in respect to genetic and environmental influences on the stability of IQ. We conclude with a discussion of the limitations of the study and avenues for future research.