In addition, it has become clear that gene expression changes drastically during development, possibly to promote appropriate maturation of the brain and other tissues. One of the gene‐regulating processes, methylation, shows well‐timed changes that allow the prediction of a subject's age (Bocklandt et al., 2011; Dongen et al., 2016; Hannum et al., 2013; Simpkin et al., 2017); recent work by ENIGMA's Epigenetics group has linked ongoing methylation to hippocampal volume and other features of brain morphometry (Jia et al., 2019). Such changes imply that different genes play a role across developmental age groups. These observations indicate that age is likely to induce heterogeneity across cohorts with age differences and that particular care must be taken when including childhood samples. Additionally, sex differences in developmental genetic association studies of both resting‐state EEG coherence and event‐related oscillations have been reported (Chorlian et al., 2017; Meyers et al., 2019), consonant with other developmental genetic studies (Cousminer et al., 2014). EEG features may also be modulated by different stages of neurological and psychiatric diseases, and these may impact the comparability of EEG recordings obtained from patients with the same disorder at different stages of disease progression (Douw et al., 2019).
