The most accessible summary can be found in "The Puzzle of Monogamous Marriage" on http://rstb.royalsocietypublishing.org/content/367/1589/657

Scroll down to section 3.a.ii (or ctrl+f search for "Implication: monogamous marriage reduces the spousal age gap, gender inequality and fertility")

Anecdotally, this is what a family friend who worked as a counselor for Southern Utah University observed repeatedly. SUU is located in Cedar City, which has one of the highest concentrations of present-day polygamists in the US.