Twin imaging studies have been useful for distinguishing the relative contributions of genetic and environmental influences toward between-person variations in brain morphology. In this report, we performed a vertex-based analysis of gray matter thickness across the entire cortex to determine the influence of genetic and environmental factors in a cohort of early adolescent twins across a narrow age window during neurodevelopment.
Supporting our hypothesis, we observed high genetic contributions toward individual differences in frontal cortical thickness, specifically the left dorsolateral and mesial superior frontal gyrus, findings that are consistent with previous reports [10
]. It has been argued that late-developing regions associated with complex cognitive functions (e.g. the prefrontal cortex) show an increase in the proportion of genetic variance with maturation, which may explain why the relative magnitude of the genetic effect in this region appears to be higher than that found in childhood cohorts, but lower than the heritability found in adult twins in previous studies [9
]. The finding of greater genetic variation in the left hemisphere than in the right hemisphere is also consistent with previous reports [9
]. Findings support a strong coupling of variations in brain morphology and genetics [18
], particularly in the frontal cortex, which may underlie the high familial liability for some diseases that are associated with frontal cortical thickness (e.g. schizophrenia) [20
Despite the use of the A/C/E model, which allows the separation of familial similarity because of common environmental factors from that because of genetic factors, and the use of a homogeneously aged group, which reduced the environmental variances, we did not find the cortical thickness of any region to show common environmental influences over 80%. The findings are consistent with previous results of children and adolescent cohorts [8
], and may suggest a greater importance of genes in the development of the brain, at least with respect to cortical thickness, especially in the frontal cortex. However, our relatively small sample size may not have sufficient power to detect the C effects, especially when they are small [22
]. Furthermore, although the ages of the adolescents included were tightly constrained, it is possible that greater variation in brain morphology may have been introduced by using a sample that is close to puberty, with variations in their level and degree of completion of development, perhaps reducing the statistical power.
In terms of unique environmental influences, our study identified the thickness of several brain regions including the superior and inferior parietal lobules to show stronger unique environmental effects. The findings of stronger unique environmental influences on these regions are consistent with previous reports [8
] and may be supported by functions linked to these regions. For example, the inferior parietal cortex has been implicated in processing spatial attention, topographic information, and language [23
]. As such experience is likely to be specific for each individual, and not shared by members of a twin pair, unique environmental factors may be more important in determining individual variations in the cortical thickness of these regions. However, the results should be interpreted with caution as measurement errors that are uncorrelated between the twins also contribute toward the estimation of unique environmental effects.