different individual choices made by male and female workers in college major, occupation, working hours, and maternal/parental leave. The remaining 6% of the gap has been speculated to originate from deficiency in salary negotiating skills and sexual discrimination. Human capital theories refer to the education, knowledge, training, experience, or skill of a person which makes them potentially valuable to an employer. This has historically been understood as a cause of the gendered wage gap but is no longer a predominant cause as women and men in certain occupations tend to have similar education levels or other credentials. Even when such characteristics of jobs and workers are controlled for, the presence of women within a certain occupation leads to lower wages. This earnings discrimination is considered to be a part of pollution theory. This theory suggests that jobs which are predominated by women offer lower wages than do jobs simply because of the presence of women within the occupation. As women enter an occupation, this reduces the amount of prestige associated with the job and men subsequently leave these occupations. The entering of women into specific occupations suggests that less competent workers have begun to be hired or that the occupation is becoming deskilled. Men are reluctant to enter female-dominated occupations because of this and similarly resist the entrance of women into male-dominated occupations. The gendered income disparity can also be attributed in part to occupational segregation, where groups of people are distributed across occupations according to ascribed characteristics; in this case, gender. Occupational gender segregation can be understood to contain two components or dimensions; horizontal segregation and vertical segregation. With horizontal segregation, occupational sex segregation occurs as men and women are thought to possess different physical, emotional, and mental capabilities. These different capabilities make the genders vary in the types of jobs they are suited for. This can be specifically viewed with the gendered division between manual and non-manual labor. With vertical segregation, occupational sex segregation occurs as occupations are stratified according to the power, authority, income, and prestige associated with the occupation and women are excluded from holding such jobs. As women entered the workforce in larger numbers since the 1960s, occupations have become segregated based on the amount femininity or masculinity presupposed to be associated with each occupation. Census data suggests that while some occupations have become more gender integrated (mail carriers, bartenders, bus drivers, and real estate