Until now evolutionary biologists have assumed that the presence of a woman's mother or her mother in law in the household increases the woman's number of children and, thus, the grandmother's number of grandchildren. Furthermore, it was assumed that women frequently move to the families and households of their husbands.
By analyzing the records of more than 2.5 million women from 14 countries worldwide, Martin Fieder and his colleagues from the Department of Anthropology of the University of Vienna found that in most cases, women do not live with either their own or their mother in law in the same household, and if they do, they have on average fewer children compared to women who only live with their spouse.
The authors suppose that two reasons may have contributed to their results: reproductive competition and competition on resources with their daughters. Particularly the latter is plausible as the analyses included many developing countries. But also reproductive competition is reasonable as a mother/mother in law in the household is particularly negatively associated with a woman's number of children if she is still in her reproductive years: a younger grandmother may have children on her own which is more important than raising her grandchildren, hence, they are competing across generations.
Publication in "Royal Society Open Science":
Huber S, Zahourek P, Fieder M.: Living with own or husband's mother in the household is associated with lower number of children: a cross-cultural analysis. 2017
R. Soc. open sci. 4: 170544.