Using the 2015 student backpack transaction data from the largest E-commerce business in China, this study takes a novel “big data” approach to test the patterns of parental sex preference by comparing the difference in cost between blue and pink backpacks at different quantiles of the backpack cost distribution. Unconditional quantile regression results show that, depending on the quantile of choice, the blue-pink difference in backpack cost can be positive, negative, or zero. This indicates the presence of son preference, daughter preference, and gender indifference in the same population. Treating backpack cost as a proxy measure of parental economic status, such results indicate that parents of high economic status invest more heavily in sons whereas parents of low economic status invest more heavily in daughters, as predicted by the Trivers-Willard hypothesis. The discovery of a third group, between the high- and low-status parents, who invest equally in sons and daughters further strengthens the argument.