Was Jean-Jacques Rousseau in favor of collectivism or individualism? How did this influence his political philosophy?
Jean-Jacques Rousseau is generally considered the quintessential collectivist. That is, he believed that the common good of the whole of society must always and everywhere outweigh the rights of individuals to make their own choices.
Rousseau began by arguing that the characteristic feature of human beings that gives us our advantage over other animals is our capacity for cooperation. This is entirely accurate; moreover, it's something I wish more people (including some biologists) better understood. Humans are smarter than other animals, but not that much smarter---our great success is due to our unmatched capacity for cooperation.
However, Rousseau believed that human beings were free and happy before the dawn of civilization, which has largely only brought us misery. This is clearly false; human standard of living has dramatically improved by any measure since the dawn of civilization. Health is the easiest to keep track of, but other measures such as economic prosperity and self-reported happiness also show an upward trend.
Based on his belief that cooperation is the key to the common good, Rousseau argued for a restructuring of society where the government would be democratic but extremely powerful, where almost every aspect of our lives would be shaped by the government forcing us to act toward the common good. He believed in an absolute majority rule by which the state even held the power over life and death by majority vote.
This is not as extreme as it may at first sound, as we do in fact have capital punishment in many democratic countries and military forces in nearly all of them. We do in that sense grant majority vote control over life and death.
Still, I think most people would object to Rousseau's extreme form of collectivism. While I think most people would agree that the common good does matter, and people cannot simply be allowed to do whatever they want even to the point of rape and murder, nonetheless most people believe that individuals deserve to have a certain amount of individual freedom to pursue their own desires and interests without interference from the rest of society. We are in this sense all both collectivists and individualists, or else something in between the two. The question is really where to find the balance---how can we achieve peace and harmony while also preserving personal liberty?