Men are more willing to donate money and volunteer for helpful causes that demonstrate virility (donating blood) and parental investment (helping children in need) when attractive women are around than when they are around other men or alone. Why? To pick up chicks, of course. Donating money and volunteering “signal” generosity, a valuable trait in a potential mate (for more examples of such showcasing, see here).
Van Vugt, M., & Iredale, W. (2012). Men behaving nicely: Public Goods as peacock tails. British Journal of Psychology. doi: 10.1111/j.2044-8295.2011.02093.x