We hypothesize that when two pathological conditions or personality traits share the same critical period for gene-environment interaction, we should expect further similarities, particularly from clinical and pathophysiological perspectives. They should therefore be considered as two facets of the same disease. To test this hypothesis we compiled data included in the Primal Health Research Database. This database (www.primalhealthresearch.com) is specialised in studies exploring correlations between what happens during the 'primal period' (fetal life, perinatal period and year following birth) and what happens later on in life in terms of health and personality traits. After mentioning the links between autism and anorexia nervosa, we explore more in depth the links between attention deficit hyperactivity disorder (ADHD) and obesity. We suggest from such examples that the nature of an environmental factor is often less important than the timing of the interaction. We conclude that the concept of gene expression, combined with Primal Health Research, might lead to reconsider conventional nosological classifications. Some previously well-defined pathological entities should be included into the framework of multifaceted diseases. On the other hand some existing pathological entities should be dismantled.