Overall, characteristics psychophysiological indices of emotional processing.such as frontal EEG asymmetry and emotion modulation of startle reflex showed somewhatlower reliability relative to measures of cognitive processing. This may seem somewhat counterintuitive because, from the evolutionary perspective, emotional response systems are older and more directly related to survival; consequently, one can expect a greater degree of genetic determination of such responses compared to e.g. executive functions the development of which may be to a larger extent mediated by social environment. On the other hand, however, developmental plasticity of emotional reactivity is an adaptive feature that permits a flexible adaptation to individually specific environment (which automatically decreases heritability at the population level). Importantly, twin research designs can provide important information about the role of environmental factors, both shared and non-shared. For example, the co-twin control studies based on the comparison of MZ twins who are discordant for a certain environmental exposure allows for a direct estimation of the effects of specific environmental factors by providing an ideal control for genetic factors and environment shared by twins.
