This article reviews the evidence for theoretical models postulating that early adversity can set in motion a deleterious developmental cascade, involving changes in hypothalamic-pituitary-adrenal (HPA) axis activity that subsequently affect the development of self-regulation in early childhood. Focusing on the first five years of life, we describe studies showing that experiences of severe deprivation or maltreatment may both lead to a hypoactive HPA axis in children. However, it is too early yet to conclude what effects exposure to adverse conditions such as poverty or family instability have on young children's HPA axis, with both low and high levels of cortisol having been observed under these circumstances. Both patterns of HPA axis dysregulation have been associated with impairments in executive functions, which are important for self-regulation. There is promising evidence that interventions targeting parenting behaviour have the potential to remediate adversity-related biological and behavioural alterations.

Disorders associated with smell loss are common in adolescents. However, current odor identification tests focus on children from age 6 and older and no cross-cultural test has to date been validated and fully implemented. Here, we aimed to investigate how 3-to-11-year-old US children performed to an adapted and shortened (11 odors instead of 14) version of a European odor identification test-the Sniffin' Kids (Schriever VA, Mori E, Petters W, Boerner C, Smitka M, Hummel T. 2014. The "Sniffin'Kids" test: a 14-item odor identification test for children. Plos One. 9:e101086.). Results confirmed that cued odor identification performance increases with age and revealed little to no differences between girls and boys. Scores below 3 and below 6 may raise hyposmia concerns in US children aged 3-7 years and 8-10 years, respectively. Even though the completion rate of the task reached the 88%, suggesting that children below age 5 were able to finish the test, their performance was relatively poor. In comparing the overall identification performance of US children with that of German children, for whom the test was specifically developed, significant differences emerged, with higher scores obtained by the German sample. Analysis of errors indicated that a lack of semantic knowledge for the olfactory-presented objects may be at the root of poor identification skills in US children and therefore constitutes a problem in the development of an odor identification test for younger children valid across cultures.

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Child maltreatment is a global phenomenon that affects the lives of millions of children. Worldwide, as many as one in three to six children encounter physical, sexual, or emotional abuse from their caregivers. Children who experience abuse often show alterations in stress reactivity. Although this alteration may reflect a physiological survival response, it can nevertheless be harmful in the long run-increasing children's disruptive behavior and jeopardizing their development in multiple domains. But can we undo this process in atrisk children? Based on several lines of pioneering research, we hypothesize that we indeed can. Specifically, we hypothesize that highly dysfunctional parenting leads to an epigenetic pattern in children's glucocorticoid genes that contributes to stress dysregulation and disruptive behavior. However, we also hypothesize that it is possible to "flip the methylation switch" by improving parenting with known-effective parenting interventions in atrisk families. We predict that improved parenting will change methylation in genes in the glucocorticoid pathway, leading to improved stress reactivity and decreased disruptive behavior in children. Future research testing this theory may transform developmental and intervention science, demonstrating how parents can get under their children's skins-and how this mechanism can be reversed.

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