I wrote a rather long article in which I show that almost every aspect of this study is flawed from both a statistics and EvPsych point of view, some of which other commenters have mentioned but several of which are new. TL;DR: RVs as defined in this paper completely ignore parenting, so can hardly explain the evolution of a parenting behaviour; the data collection is poor for doing proper statistics (hence they don’t) so the total population count is not a good measure of the error; and it’s easy to get absurdly high correlations between two time series because they autocorrelate, which is why you should never do this sort of analysis even under a “correlation is not causation” gloss. https://scienceisshiny.wordpress.com/2020/09/11/everything-wrong-with-the-paper-human-grief-is-its-intensity-related-to-the-reproductive-value-of-the-deceased/
I wrote a rather long article in which I show that almost every aspect of this study is flawed from both a statistics and EvPsych point of view, some of which other commenters have mentioned but several of which are new. TL;DR: RVs as defined in this paper completely ignore parenting, so can hardly explain the evolution of a parenting behaviour; the data collection is poor for doing proper statistics (hence they don’t) so the total population count is not a good measure of the error; and it’s easy to get absurdly high correlations between two time series because they autocorrelate, which is why you should never do this sort of analysis even under a “correlation is not causation” gloss. https://scienceisshiny.wordpress.com/2020/09/11/everything-wrong-with-the-paper-human-grief-is-its-intensity-related-to-the-reproductive-value-of-the-deceased/