Abstract—: Spontaneous electrical brain activity is considered as a state of tonic activation or inhibition, which is the physiological basis for normal brain function and pathology. Projections of the serotonergic system are widely widespread in the cerebral cortex and affect both the processes of neuroplasticity and the formation of fundamental neural networks that determine the brain activity at rest. The STin2VNTR gene appears to be an allele-dependent enhancer of the expression of the serotonin transporter gene. This study was conducted to identify the associations of STin2VNTR polymorphism with the characteristics of background EEG in healthy older (51 men (64.5 ± 7.13 years of age) and 48 women (63.56 ± 5.4 years)) and younger (86 men (22.5 ± 3.5 years) and 110 women (21.17 ± 2.7 years)) adults. The endophenotypes of brain electrical activity associated with the STin2VNTR polymorphism of the serotonin transporter gene are characterized by specific patterns of α3 power asymmetry in parietal–occipital regions of the brain hemispheres and differ in younger and older subjects. In the younger age group, the 12'/12' genotype carriers had significantly higher values of the asymmetry index of the α3 power (right > left) in the parietal–occipital regions as compared with other genotypes. The presence of allele 12 in the genotype positively correlates with the value of asymmetry index of the α3 rhythm. In young carriers of allele 12 (10'/12' and 12'/12'), lateral differences were statistically significant. In the older age group, the 10'/10' genotype carriers had the lowest values of asymmetry index; only the carriers of this genotype had statistically significant lateral differences due to higher rhythm power in the parietal–occipital region of the right hemisphere as compared to the left one. Lateral differences due to high α-rhythm power values in the posterior region of the right hemisphere may be considered as electrophysiological endophenotype and may be associated with predisposition to the possible occurrence of schizoid disorders in young carriers of the 12'/12' genotype.