Furthermore, large income disparities can hold back economic growth by weakening aggregate demand (United Nations, 2013). The widening of income gaps between employees with differing education levels in the 1980s and 1990s was something of a surprise, considering the expected labour supply and demand trends. On the one hand, the expansion of education systems increased the supply of better qualified personnel. On the other, it was expected that liberalizing economic reforms would benefit the sectors that mainly used more plentiful factors of production, especially low- and medium-skilled labour. The outcome of these processes would have been to boost the wages of the low- and medium-skilled workers. Of the multiple factors that could have contributed to this trend, it was not possible to identify any single key cause, it being considered that a series of interactions are likely to be responsible (Fishlow and Parker, 1999).