I found this very strange as well, so I skipped down to the section that discussed this:
> In countries that have undergone the demographic transition, twin, adoption and family studies have pointed to a substantial genetic effect on fertility (Fisher, 1930; Kirk et al., 2001; Kohler, Rodgers, & Christensen, 1999; Murphy, 1999; Murphy & Knudsen, 2002; Rodgers, Hughes, et al., 2001a; Rodgers, Kohler, et al., 2001b). For example, Fisher (Fisher, 1930) found that a woman could expect 0.21 additional children for each additional child that her mother had, and 0.11 additional children for each additional child that her grandmother had. From this, Fisher suggested that the heritability of fertility at that time was 0.4 (40% of the variation in fertility is explained by genetic factors). Summarising research conducted through to 1999, Murphy (Murphy, 1999) noted that the heritability of fertility averaged around 0.2 in post-demographic transition societies, with the estimates increasing in recent periods. Kohler et al. (Kohler et al., 1999) examined data on Danish twins born in the periods 1870 to 1910 and 1953 to 1964. The first period covers the demographic transition and the second the end of the baby boom. In the first cohort, the heritability of fertility in women varied from close to zero in the pre-transition period to as high as 0.4 to 0.5 during the demographic transition. Estimates of heritability remained strong for the 1953 to 1964 cohort. From an analysis of data for Danish twins from the 1950s, Rodgers et al. (Rodgers, Kohler, et al., 2001b), attributed slightly more than one quarter of the variation in fertility to genetic factors. Rodgers and Doubty (Rodgers & Doughty, 2000) found a median heritability of 0.33 in a contemporary United States population, and heritabilities for underlying desires, ideals and expectations ranging between 0.24 and 0.76. Where measured, the variance attributable to shared environment in low-fertility populations was generally lower than the genetic effects (Kirk et al., 2001; Kohler et al., 1999; Kohler, Rodgers, Miller, Skytthe, & Christensen, 2006; Zietsch, Kuja-Halkola, Walum, & Verweij, 2014). However, the relative balance of genetic and shared environmental factors can change quickly over time (Kohler et al., 1999; Kohler et al., 2006).