In the early twentieth century in the United States and other Western countries, women were much less likely than men to smoke cigarettes, due in part to widespread social disapproval of women's smoking. During the mid-twentieth century, growing social acceptance of women's smoking contributed to increased smoking adoption by women. Increased social acceptance of women's smoking was part of a general liberalization of norms concerning women's behavior, reflecting increasing equality between the sexes. These historical trends were due in part to increases in women's employment. However, in the contemporary period employment appears to have little or no effect on women's smoking. Sex role norms and general expectations concerning gender-appropriate behavior have had a variety of effects on gender differences in smoking. First, general characteristics of traditional sex roles, including men's greater social power and generally greater restrictions on women's behavior, contributed to widespread social pressures against women's smoking. Second, traditional sex role norms and expectations have fostered gender differences in personal characteristics and experiences which influence smoking adoption. For example, rebelliousness has been more expected and accepted for males, and greater rebelliousness among adolescent males has contributed to greater smoking adoption by males. Finally, certain aspects of sex roles have contributed to gender differences in appraisal of the costs and benefits of smoking. For example, physical attractiveness is emphasized more for females and the contemporary beauty ideal is very slender, so females are more likely to view weight control as a benefit of smoking. Several other hypotheses concerning the causes of gender differences in smoking are not supported by the available evidence. For example, it appears that women's generally greater concern with health has not contributed significantly to gender differences in the prevalence of smoking. Similarly, it appears that sex differences in physiological responses to smoking have made only minor contributions to gender differences in smoking adoption or cessation.