The PRI maintained absolute power over the country for most of the twentieth century: besides holding the Presidency of the Republic, until 1976 all members of the Senate belonged to the PRI, while all of the state governors were also from the PRI until 1989. Throughout the seven decades that the PRI governed Mexico, the party used a combination of corporatism, co-option and (at many times, violent) repression to hold power, while usually resorting to electoral fraud when these measures were not enough. In particular, the presidential elections of 1940, 1952 and 1988 were characterized by massive irregularities and fraudulent practices denounced by both domestic and international observers. While during the early decades of PRI rule Mexico benefited from an economic boom which improved the quality of life of most people and guaranteed political and social stability, issues such as inequality, corruption and the lack of democracy and political freedoms cultivated growing resentment against the PRI, culminating in the 1968 Tlatelolco massacre in which the Army killed hundreds of unarmed student demonstrators. In addition, a series of economic crises beginning in the 1970s drastically lowered the living standards of the population.