## 1 Introduction
The discovery of the Flynn effect has spawned many studies trying to determine the factors that influence the variability of average human intellectual abilities over time. Genetic factors, environmental factors, decreased fertility, and methodological problems that distort measurements were previously considered to explain the increase in IQ over time [24]. Investigating the Flynn effect, [PERSON] and [PERSON] proposed a formal model that explained how the environment and genes can interact and influence IQ [25]. In the course of numerous studies, it was found that environmental factors predominate in the nature of the [PERSON] effect [26]. This is supported by the observation that the [PERSON] effect is most pronounced when applied to the tasks that have the least load on the g factor [27]. The [PERSON] effect has been summarized in meta-analytic studies. The authors of [28] determined that the Flynn effect and group differences in IQ indices were explained by different reasons. The authors of [26] discuss the stability of the Flynn effect was confirmed in different age groups, measures, samples, and levels of performance. A meta-analysis of data collected over the 1950-2014 period in 48 countries found the [PERSON] effect to be present in every age group, and the Flynn effect was stronger in developing countries. The authors of [29] found a decrease in IQ growth in the last decade of the 20 th century, and a positive relationship between full-scale, crystallized, and spatial IQ with changes in the gross domestic product per capita was revealed (the change in the gross domestic product showed negligible effects for fluid IQ). The meta-analysis was performed based on 219 studies in a sample over 105 years (1909-2013) in 31 countries. Russia was not included in this sample. One of the first large-scale studies of the [PERSON] effect in Russia was presented in 2019 [30]. To analyze the dynamics of intellectual abilities, 238,363 protocols of voluntary online testing for 2012-2018 were used. The participants in the experiment were presumably men born in 1974-1999. The analysis of these data showed that for respondents born until the mid-1980s, there was a decrease in IQ test results replaced by a linear increase, the rate of which was estimated at about 0.19 IQ points per year. The obtained dynamics of IQ test results could be explained by the peculiarities of the socio-economic situation in Russia in the 1980s-1990s. At the beginning of the 21 st century, scientists began to observe a slowdown or reverse the development of the Flynn effect. [PERSON] and [PERSON] demonstrated a sharp drop in average IQ values in a large sample spanning three decades of Norwegian birth cohorts (1962-1991) [31]. This phenomenon is called the \"negative Flynn effect\" [32; 33]. In the study of the reasons for the decrease in the population IQ, such factors were revealed as immigration [31; 34], dysgenics [31; 35], a change in the sex ratio of the sample [31], and education standards decline [31; 36]. Thus, the conclusions drawn from the above studies are very diverse. However, the general result of the research can be formulated as follows: The Flynn effect is not universal and is sensitive to environmental factors. This is confirmed by numerous studies which showed that during the twentieth century, there was a rather significant increase in IQ indices, and since the late 20 th-early 21 st centuries, the opposite effect of weakening the intelligence, especially its individual components, can be observed. At the same time, despite a huge amount of research, there is no consensus on the reasons for an increase or decrease in the average human intellectual abilities over time. The existing studies on the issue under consideration are very different in terms of nationality, age, studied subjects, sample sizes, composition, and completeness of the intelligence tests, as well as methods of data analysis.