1. Leonard, H. & Wen, X. The epidemiology of mental retardation: challenges and opportunities in the new millennium. Ment. Retard. Dev. Disabil. Res. Rev. 8, 117–134 (2002). A comprehensive meta-analysis on the aetiology and prevalence of mental retardation in males and females.
2. Polder, J. J., Meerding, W. J., Koopmanschap, M. A., Bonneux, L. & van der Maas, P. J. The cost of sickness in the Netherlands in 1994. Ned. Tijdschr. Geneesk. 142, 1607–1611 (1998). A comparative evaluation of disease-related costs, concluding that mental retardation is the most important cost factor in health care.
3. Penrose, L. S. A Clinical and Genetic Study of 1280 Cases of Mental Defect Vol. 229 (HMSO, London, 1938).
4. Lehrke, R. A theory of X-linkage of major intellectual traits. Am. J. Ment. Defic. 76, 611–619 (1972).
5. Lehrke, R. G. X-linked mental retardation and verbal disability. Birth Defects Orig. Artic. Ser. 10, 1–100 (1974). References 4 and 5 are influential early publications that postulate an important role for X-linked genes in cognition and mental retardation.