In attempting to explain the delayed starts of menstruation and sexual activity, the researchers ruled out factors such as socioeconomic class. Instead, they conjectured that older brothers may have delayed the physiological maturation of sisters by absorbing more parental resources or by applying psychological stresses. Meanwhile, younger brothers may have delayed girls' behavioral maturation by demanding that their older sisters assume care-taking roles. 　　

為了更好地解釋為何女孩初潮以及第一次性關(guān)系發(fā)生時間會因兄弟的存在而推遲，研究人員排除了某些因素如社會經(jīng)濟等級因素。相反地，他們推測兄長吸取了更多的父親元素且能夠施加心理壓力，故而使得女孩子月經(jīng)初潮推遲。而弟弟希望姐姐承擔(dān)起照顧自己的責(zé)任，從而延遲了女孩們行為的成熟。