We are greatly appreciative of the support from the twins and their families, and their willingness to participate in our study. We thank Marlene Grace, Ann Eldridge and Natalie Garden for participant recruitment and data collection at QIMR Berghofer Medical Research Institute. We also thank the Queensland Curriculum and Assessment Authority for the provision of QCST data and statistics. Any analyses of QCST data constitutes our own assessment of the data and is not a representation of the agency. This collaborative research was supported by the Human Frontier Science Program (grant number RG0154/1998B) and grants from the Australian Research Council (A79600334, A79906588, A79801419, DP0212016, DP0664638, DP1093900) and the Netherlands Organisation for Scientific Research (NWO) 016.Veni.198.058. L.C-C. was supported by a QIMR Berghofer fellowship.

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