Jones, B.C., Little, A.C., Boothroyd, L., DeBruine, L.M., Feinberg, D.R., Smith, M.J. Law, Cornwell, R.E., Moore, F.R., Perrett, D.I., "Commitment to relationships and preferences for femininity and apparent health in faces are strongest on days of the menstrual cycle when progesterone level is high", Hormones and Behavior, 2005