reverse became true. Some cultural groups continued to place a value on the moral purity of an abstainer, but abstinence was caught up in a wider reevaluation of moral values. During the early 20th century, prominent feminist and birth control advocate Margaret Sanger argued that abstinence from sexual activity led to greater endurance and strength, and was a sign of the best of the species: "Though sex cells are placed in a part of the anatomy for the essential purpose of easily expelling them into the female for the purpose of reproduction, there are other elements in the sexual fluid which are the essence of blood, nerve, brain, and muscle. When redirected into the building and strengthening of these, we find men or women of the greatest endurance and greatest magnetic power. A girl can waste her creative powers by brooding over a love affair to the extent of exhausting her system, with the results not unlike the effects of masturbation and debauchery." J. D. Unwin was a British ethnologist and social anthropologist at Oxford University and Cambridge University. Unwin wrote several books including Sex and Culture, (1934). In Sex and Culture Unwin studied 80 primitive tribes and 6 known civilizations through 5,000 years of history and found a positive correlation between the cultural achievement of a people and the amount of sexual restraint which they observed. The author finds that the most culturally successful groups always exhibit lifelong monogamous relationships which include sexual abstinence outside of marriage. According to Unwin, after a nation becomes prosperous it becomes increasingly liberal with regard to sexual morality and as a result loses its cohesion, its impetus and its purpose, ultimately having a negative effect on society: "The whole of human history does not contain a single instance of a group becoming civilized unless it has been absolutely monogamous, nor is there any example of a group retaining its culture after it has adopted less rigorous customs." During fertile period Sexual abstinence can be practiced during the period in which the woman is fertile. Before marriage Premarital chastity In most cultural, ethical, and religious contexts, sex within marriage is not considered to be contrary to notions of chastity. Some religious systems prohibit sexual activities between a person and anyone other than a spouse of that person, as have past legal systems and societal norms. In such contexts, sexual abstinence was prescribed for unmarried