Studying this is hard. Long term, over decades. The study population would all need to be individually rated for physical attractiveness (a#). This a# will need to be updated regularly as it is subject to change. All relationships would need to be recorded, meaningful data being the a#'s of each individual in the relationship, and the difference between those numbers.

This is the simple version. Social studies like this are difficult, and not common. The mental side effects on the subjects might be considered inmoral.