Mapping the human genome has provided the foundation for studies investigating the role of genetic and environmental factors in the aetiology of complex diseases and neurological disorders. Assessment of genetic risk factors is becoming increasingly comprehensive with the advent of high-throughput, genome-wide approaches. The constancy of genotypes has enabled their use as risk factors in case–control and related hybrid study designs. These genetic risk factors not only carry a low misclassification potential, but they also promise to result in epidemiological studies focused on individuals at high risk of developing chronic complex disease.

Exposure ascertainment for complex diseases largely relies on participants to recall past lifestyle behaviours and diet. Poor recall, especially differential recall between cases and controls, remains a fundamental concern threatening the validity of epidemiological studies. As we embark on studies evaluating exposure throughout human life, such as testing for the developmental origins of adult disease,123 we can anticipate misclassification resulting from poor recall to worsen. A growing body of evidence now suggests that epigenetic features of the genome that regulate phenotype without changing the nucleotide sequence may provide …