Male workers are more likely to be members of trade unions, thus enjoying the advantages of collective bargaining for wages and earning more than women (Flaherty & Caniglia, 1992). Overall, women are about half as likely to be union members as men. Rinehart wrote, ¡§ This accommodation to capitalist power was formalized in what is known as the post ¡V World War II compromise forged in the midst of the intense class struggles of the 1940s. This settlement between labor, big capital, and the state featured the establishment of a new industrial relations system and stipulated a set of trade ¡V off. Unions were legally recognized and accorded organizational security.¡ (P.184).

             Changing in the education level of the population is related to the income distribution. In the U.S, the financial return to a university or college degree also declined during the 1970s. One explanation of this phenomenon was the substantial increase the proportion of the population going to university, particularly the entry into the labor force of the ¡§ Baby-Boom¡ generation during the 1970s (Welch 1979). Freeman (1976, 1980) argued that the demand for educated workers also declined, so that not all of the change in relative earnings could be attributed to temporary developments on the supply side. Dooley (1986) concluded that the entry of the large baby¡Vboom cohort during this period did lower earnings growth for this group, but that this demographic effect could not account for the observed harrowing of earnings differentials by level of education. Dooley¡s results thus suggest that demand ¡V side forces may also have played a role.

             Changing in the age structure of the population is related to the income distribution. The shapes of the earning streams reflect the main key factors which is earnings increase with age but at a decreasing rate. This concave shape reflects the fact that individuals generally continue to make human capital investments in the form of on-the-job training and work experience once they have entered the labor force.