The hidden curriculum refers to the unspoken, yet apparently prevalent inclusion of views that tend to support the continued existence and maintenance of the dominant or upper class. Critical theory advocates that to prevent the advancement of the hidden curriculum to the detriment of the lower classes, schools use officially sanctioned textbooks that are unbiased regarding views that will promote or maintain the dominant or upper class. Also, teachers are expected to encourage students to voice their beliefs about their values, rather than those that are simply popular.