## Economic and social background has an influence on the development of intellect

A study on the effects of child bullying on gene expression involved 28 pairs of identical twins, one of which was always a victim of bullying and the other was not. Serotonin is a neurotransmitter that helps transmit nerve impulses, and as a hormone, it causes vasoconstriction and affects sleep and thermoregulation. Serotonin is generally considered the hormone of happiness and well-being. Deficiency of serotonin manifests itself in depressed mood, sleep disturbances, and depression. For this research, the gene studied codes for the serotonin transporter (SERT). The result of the study was that the DNA methylation level of the SERT gene was increased in the bullied twin (in comparison with the non-bullied sibling). The consequence of the increased methylation is the weak production of the transporter that has to mediate the entry of serotonin into the cells (Figure 7.2). At the same time, a change in the response to cortisol during periods of stress was also observed. This is another of the studies demonstrating that early stress exposure has effects on the state of the epigenome, alters stress tolerance, and can significantly affect child development.

Economic background can also have an impact on the epigenome. In 2017, a published study focused on a sample of 1,000 girls who were divided into "low-income families" and "high-income families" groups based on the household in which they lived. Differences in brain development were found, that was most evident in poorer girls who lived in neighbourhoods where they were exposed to wealthier families. Such differences manifested as a thinning of the cerebral cortex, which is a commonly measured indicator of brain tissue development. The cerebral cortex is where conscious activity takes place. Expression of genes encoding the glucocorticoid receptor (NR3C1) and the androgen receptor (AR) was suppressed in girls from poorer backgrounds. These two receptors respond to stress hormones and are associated with the occurrence of depression. Interestingly, boys showed no such disparity, which may be explained by the fact that they have higher levels of testosterone, which can block hormones activated by social stress.