The 'classical twin design' (CTD) circumvents parameter indeterminacy by assuming (1) negligible higher-order epistasis; and (2) either nonadditive genetic or common environmental effects are nonexistent, creating two potential sources of bias (Eaves et al., 1978; Grayson, 1989). Because the extended 'twin-family design' (ETFD) uses many more unique covariance observations to estimate parameters, common environmental and nonadditive genetic parameters can be simultaneously estimated. The ETFD thereby corrects for what is likely to be the largest of the two sources of bias in CTD parameter estimates (Keller & Coventry, 2005). In the current paper, we assess the extent of this and other potential sources of bias in the CTD by comparing all published ETFD parameter estimates to CTD parameter estimates derived from the same data. CTD estimates of the common environment were lower than ETFD estimates of the common environment for some phenotypes, but for other phenotypes (e.g., stature in females and certain social attitudes), what appeared as the common environment was resolved to be assortative mating in the ETFD. On average, CTD estimates of nonadditive genetic factors were 43% lower, and additive genetic factors 63% higher, than ETFD estimates. However, broad-sense heritability estimates from the CTD were only 18% higher than ETFD estimates, highlighting that the CTD is useful for estimating broad-sense but not narrow-sense heritability. These results suggest that CTD estimates can be misleading when interpreted literally, but useful, albeit coarse, when interpreted properly.

It is well established that major depressive disorder (MDD) is partly heritable. We present a genome-wide linkage study aiming to find regions on the genome that influence the vulnerability for MDD. Our sample consists of 110 Australian and 23 Dutch pedigrees with two or more siblings affected with MDD (total N = 278). Linkage analysis was carried out in MERLIN. Three regions showed suggestive linkage signals. The highest LOD-score of 2.1 was found on chromosome 17 at 52.6 cM along with LOD scores of 1.9 and 1.7 on chromosome 8 at 2.7 cM and chromosome 2 at 90.6 cM, respectively. The result on chromosome 8 seems most promising as two previous studies also found linkage in this region, once suggestive and once significant. The linkage peak on chromosome 17 harbors the serotonin transporter gene. In the Australian and Dutch sample, the serotonin transporter length polymorphism did not show evidence for association, thus other genes in this region or other polymorphisms in the serotonin transporter gene might be associated with MDD. Further replication is needed to establish the relevance of our linkage finding on chromosome 2.

Early literacy and language skills of twin children in the USA, Australia, and Scandinavia were explored in a genetically sensitive design (maximum N = 615 pairs). For this article, we report aspects of preschool and Grade 2 data. In Grade 2, there were strong genetic influences on word reading, reading comprehension, and spelling. Vocabulary was about equally affected by genes and shared environment. Multivariate analyses indicated substantial genetic overlap among the Grade 2 literacy variables. Longitudinal analyses showed that genetic factors evident at the preschool stage continued to affect literacy and vocabulary three years later in grade 2, but there was also evidence of new genetic factors evident at the preschool stage continued to affect literacy and vocabulary three years later in Grade 2, but there was also evidence of new genetic factors coming into play over the time interval, at least for literacy. Suggestions are made about the search for underlying biological and cognitive processes, and educational implications are explored.

To date, research shows that the genetic etiology of reading disability is not dissimilar to that observed for the normal range, supporting the generalist genes hypothesis (Plomin and Kovas 2005). However, findings on the genetic etiology of reading disability in boys versus girls are mixed. Some observe greater heritability in boys (Harlaar et al. 2005; Stevenson 1992), while others do not (Wadsworth and DeFries 2005). We explored these issues for reading measured with the TOWRE at Grade 1 with a dataset compiled across Australia and the US. The full distribution of the sample comprised 413 MZs and 420 DZs. The top and bottom probands were those with scores greater than 1 SD either above or below the mean. For the bottom proband, the estimates of A, D, C and E were 53, 0, 26 and 21%; for the full distribution they were 77, 0, 7 and 16%; and for the top proband they were 72, 17, 0 and 11%. Through not significant, this shows a trend whereby, when explaining differences between high end reading ability and the normal range, genetic effects were more important, but when explaining differences between reading disability and the normal range, environmental effects played more of a role. While inconsistent with previous research, our trend may be from detrimental environmental effects that impact low but not high end reading ability, rather than differential genetic effects, so our results are not inconsistent with the generalist genes hypothesis. An analysis of the bottom proband separately for males and females showed slightly stronger genetic effects in males (effects of A, C and E were 45, 21 and 24%) than females (effects of A, C and E were 63, 21 and 16%). These differences were not significant thought were in the same direction as Harlaar et al. (2005) and Stevenson (1992) but not Wadsworth and DeFries (2005).

Each year, Australian students in Grades 3, 5, 7, and 9 sit nationwide tests in literacy and numeracy. These tests inform government, principals, and parents about student, school, and state performance in five domains: reading, spelling, grammar and punctuation, writing, and numeracy. As such, the results of these tests are of wide interest for diverse reasons depending on the stakeholder in question. In this thesis I examine the influence of genes and the environment on individual differences in performance on these tests. Using longitudinal data collected from a large sample of Australian twins and their siblings.

Initially, as a test of validity, I compared the performance of large-scale reading tests against three literacy tests in comprehension, word reading and vocabulary individually administered to twins in Grade 3. The individually administered tests accounted for a substantial amount of the variance in the large-scale reading tests. Additionally, they were preferentially related, both genetically and environmentally, to large-scale reading tests compared to large-scale numeracy tests, confirming that large-scale school reading tests measure, at least in part, the literacy skills tapped by individual tests considered “gold-standard” in testing.

In the second paper, I examined the extent to which genes and the environment contributed to variation in and covariation among the five domains in each grade. Averaged across domains and grade, genetic factors explained 60%, shared environment 10%, and unique environment 30% of the variation. Independent pathway models showed similar genetic and environmental structures at each grade with approximately one third to one half of the variation in each domain due to genes that influenced all domains.

In the third paper, I explored the genetic and environmental influences on stability and growth in each of the domains. Stability in performance was primary due to genes. For growth, reading followed a compensatory growth pattern, and variation in growth was due to the genes that also influenced differences in performance at initial testing. By contrast, growth in numeracy was principally influenced by unique environmental factors. These results suggest individual differences in growth of reading are primarily due to a genetically influenced developmental delay in the acquisition of necessary skills, while for numeracy, differences are due to environmental influences, such as different teachers or interests.

In the fourth paper, I tested if family or school SES moderated heritability of performance. Genetic influence was substantial and stable across all levels of family and school SES, with some evidence of a stronger influence of the shared environment when SES was lower, particularly for Grade 3 literacy. A final chapter presents a discussion summarising the principal findings, their implications, and their limitations.

The classical twin design (CTD) is the most common method used to infer genetic and environmental causes of phenotypic variation. As has long been acknowledged, different combinations of the common environment/assortative mating, and additive, dominant, and epistatic genetic effects can lead to the same observed covariation between twin pairs, meaning that there is an inherent indeterminacy in parameter estimates arising from the CTD. The CTD circumvents this indeterminacy by assuming that higher-order epistasis is negligible and that the effects of either dominant genetic variation or the common environment are nonexistent. These assumptions, however, lead to consistent biases in parameter estimation. The current paper quantifies these biases and discusses alternative strategies for dealing with parameter indeterminacy in twin designs. One strategy is to model the similarity among other relatives in addition to twins (extended twin-family designs), which reduces but does not eliminate indeterminacy in parameter estimates. A more general strategy, applicable to all twin designs, is to present the parameter indeterminacy explicitly, as in a graph. Presenting the space of mathematically equally likely parameter values is important, not only because it aids the proper interpretation of twin design findings, but also because it keeps behavioral geneticists themselves mindful of methodological assumptions that can easily go unexamined.

Studies using the classical twin design often conclude that most genetic variation underlying personality is additive in nature. However, studies analyzing only twins are very limited in their ability to detect non-additive genetic variation and are unable to detect sources of variation unique to twins, which can mask non-additive genetic variation. The current study assessed 9672 MZ and DZ twin individuals and 3241 of their siblings to investigate the environmental and genetic architecture underlying eight dimensions of personality: four from Eysenck’s Personality Questionnaire and four from Cloninger’s Temperament and Character Inventory. Broad-sense heritability estimates from best-fitting models were two to three times greater than the narrow-sense heritability estimates for Harm Avoidance, Novelty Seeking, Reward Dependence, Persistence, Extraversion, and Neuroticism. This genetic non-additivity could be due to dominance, additive-by-additive epistasis, or to additive genetic effects combined with higher-order epistasis. Environmental effects unique to twins were detected for both Lie and Psychoticism but accounted for little overall variation. Our results illustrate the increased sensitivity afforded by extending the classical twin design to include siblings, and may provide clues to the evolutionary origins of genetic variation underlying personality.