However women are still subjected to forms of sexual harassment, which ranges form jokes to threats. The most common form of sexual harassment is “hostile environments” which aims to make women feel unsafe and uncomfortable. This sexual harassment serves as a means of men enforcing their superiority over women, and it is one of the most prominent forms of gender inequality in the workplace  Some research has found that, in classroom settings, male students tend to talk more, and longer, than female students. This was determined to be particularly noticeable when the instructor is male. Similar results were found previously in hospitals by Erving Goffman in 1961, university discussion groups by Elizabeth Aries in 1972, and in corporate settings by Rosabeth Kanter in 1977. Women and men experience different types of mobility within the workplace. For example, women tend to experience a glass ceiling, an invisible barrier that prevents them from moving up the corporate ladder. An example of this is a study from Sweden that compared the number of females in director jobs to men in director jobs. The study showed that there were statistically more men than women holding those jobs and showed that the results were also shown in other countries such as the USA. Men in jobs traditionally held by women, such as nursing, elementary school teaching, and social work, experience a "glass escalator" effect in which they are able to quickly ascend the job hierarchy to become managers and principals. There also tends to be a gender pay gap between men and women, with women earning 77% as much as men. One cause of the gender pay gap may be due to occupational segregation, which pushes men and women towards gender-specific forms of employment, rather than pay discrimination. Another possible cause is the double burden, a phenomenon in which women perform most of the unpaid childcare and household work despite being otherwise employed for pay. A third possible cause is occupational sexism, one part of which favors men for promotions due to their traditional breadwinner status. The 2001 class action lawsuit, Dukes v. Wal-Mart Stores, Inc., charged Wal-Mart with sexist hiring and promotion practices. There were also studies done that showed that having women in higher paying positions would correlate to an increased chance of depressive symptoms. These studies talked about how the depression was caused by the negative social experiences at their job, such as social isolation and negative social interactions, that weaken the mental health of the women in authority positions. Of which referenced that men had more value for their status in the work place than women. These social interactions would have been caused by cultural gender norms.