By observing the large negative exogenous shock to the French male population from to WWI casualties, we study the effect of a change in the sex ratio on marital assortative matching by social class. First, we analyzed a novel data set that links marriage-level to French population and military mortality. Then, we calculated the sex ratio in a region with military mortality, which exhibits exogenous geographic variation. Ultiamtely, we found that men married women of higher social class than themselves more often in regions that experienced a larger decrease in the sex ratio. A decrease in the sex ratio of man to woman from 1.00 to 0.90 increased the probability that men married up by 8 percent. These findings provide insight into individuals’ preferences for spouses. Men appear to prefer to marry higher-class spouses, but cannot do so when the sex ratio is balanced.