Michael A. Alexander:
"Inequality cycles associated with secular cycles have been identified in pre-industrial societies (Turchin and Nefedov 2009:36, 82). A secular cycle is a “demographic-social-political oscillation of very long period (centuries long)” (Turchin and Nefedov 2009:5). They arise from population cycles (Korotayev et al. 2006). In an agrarian economy, demand for labor is ultimately limited by the maximum amount of arable land, while labor supply is proportional to population. As the fraction of arable land under cultivation approaches one, rising population means a rising labor supply relative to an increasingly fixed labor demand which leads to lower real wages and rising economic inequality. Thus, population and inequality trends are correlated, either can serve to define a secular cycle. Several models that describe how population/economic inequality affects elite number, state strength and sociopolitical instability in agrarian societies have been proposed (Turchin 2003:123; Turchin and Korotayev 2006:122; Turchin 2013:251). Some of these have been shown to give a good fit of historical data (Alexander 2016). The empirical and theoretical methods developed for agrarian societies do not apply to industrial societies."
Michael A. Alexander:
"Turchin (2016) has recently tried to apply the secular cycle concept to America. Although there are no population cycles, there are still empirical cycles in economic inequality that may be used to define secular cycles. Turchin uses measures of economic, physical, and social well-being: relative wage (wage/GDP per capita), male stature, life expectancy, and age at first marriage as proxies for inequality. A composite of these measures defines the American secular cycles in terms of inequality (Turchin 2016: 73,fig. 3.7).The composite trends show two secular cycles: the first over 1780–1930 the second from1930 to the present(Turchin 2016: 73).He develops a modified demographic explanation for inequality in which demand for labor is assumed to be independent of labor supply, allowing separate relations for each to be developed and their ratio used to explain real wage trends. Changes in labor supply, chiefly from changes in immigration, depress real wages, causing rising inequality. Given this, he employs percent foreign born as another proxy for inequality."