Review of twin and family studies on neuroanatomic phenotypes and typical neurodevelopment

Abstract

This article reviews the extant twin studies employing magnetic resonance imaging data (MRI), with an emphasis on studies of population-based samples. There have been approximately 75 twin reports using MRI, with somewhat under half focusing on typical brain structure. Of these, most are samples of adults. For large brain regions such as lobar volumes, the heritabilities of large brain volumes are consistently high, with genetic factors accounting for at least half of the phenotypic variance. The role of genetics in generating individual differences in the volumes of small brain regions is less clear, mostly due to a dearth of information, but rarely because of disagreement between studies. Multivariate analyses show strong genetic relationships between brain regions. Cortical regions involved in language, executive function, and emotional regulation appear to be more heritable than other areas. Studies of brain shape also show significant, albeit lower, genetic effects on population variance. Finally, there is evidence of significant genetically mediated relationships between intelligence and brain structure. At present, the majority of twin imaging studies are limited by sample sizes small by the standards of behavioral genetics; nevertheless the literature at present represents a pioneering effort in the pursuit of answers to many challenging neurobiological questions.

Figure 1. Summary of heritability estimates for volumes of large neuroanatomic structures

Note: Reports are listed in parentheses if heritability is calculated via Falconer estimation, either by the authors of the study or post hoc based on correlations; the remaining studies use SEM. * = Pediatric studies ICV = intracranial volume Brain = total brain or total cerebrum LH = left hemisphere RH = right hemisphere GM = total gray matter WM = total white matter

Figure 2. Sample characteristics of the extant MRI twin studies on typical populations

Note: To facilitate comparisons with singleton and sibling subsamples, individual twins are counted rather than twin pairs. Several reports share overlapping samples, either derived from the National Heart, Lung, and Blood Institute sample (1), the Netherlands Twin Registry (2), or NIMH’s longitudinal pediatric imaging study (3). * = reports based on data from pediatric populations.

Figure 3. Sample characteristics of the extant MRI twin reports on neuropathology

Note: Control twins are shown as solid colors (MZ white, DZ black). Qualitative characteristics of these studies are provided in Table 2.