career." Furthermore, statistics had shown that "only 17 percent of the women who are full professors of engineering have children, while 82 percent of the men do." Attempts in equalizing household work Despite the increase in women in the labor force since the mid-1900s, traditional gender roles are still prevalent in American society. Many women are expected to put their educational and career goals on hold in order to raise a family, while their husbands become primary breadwinners. However, some women choose to work and also fulfill a perceived gender role of cleaning the house and caring for children. Despite the fact that certain households might divide chores more evenly, there is evidence supporting the issue that women have continued being the primary care-giver in family life even if they work full-time jobs. This evidence suggests that women who work outside the home often put an extra 18 hours a week doing household or childcare related chores as opposed to men who average 12 minutes a day in childcare activities. One study by van Hooff showed that modern couples, do not necessarily purposefully divide things like household chores along gender lines, but instead may rationalize it and make excuses. One excuse used is that women are more competent at household chores and have more motivation to do them, and some say the jobs men have are much more demanding. In The Unsettling of America: Culture and Agriculture, Wendell Berry wrote in the 1970s that the "home became a place for the husband to go when he was not working ... it was the place where the wife was held in servitude." A study conducted by Sarah F. Berk, called "The Gender Factory", researched this aspect of gender inequality as well. Berk found that "household labor is about power". The reason the spouse performing less housework is not the spouse in power is simple; they have more free time than their counterpart; therefore, they are able to do more of what they want after the average workday. Gender roles have changed drastically over the past few decades. In an article taking the time period of 1920–1966, data was recorded which surmised that women spent most of their time tending the home and family. A study assessing changing gender roles between males and females showed that as women begin to spend less time in the house, men are taking over the role