Social stratification refers to the existence of structured inequalities between different groups in society in terms of their access to material or symbolic rewards, such as wealth, income, prestige, power, status, and opportunities. While some forms of social differentiation may exist in all societies, significant and enduring patterns of inequality, leading to hierarchical arrangements of social groups, tend to arise with the development of more complex, state-based systems. The most distinctive and extensively studied form of stratification in modern societies involves class divisions, based primarily on economic factors, but stratification can also occur along lines of race, ethnicity, gender, age, sexual orientation, and other social categories. Sociologists examine the causes and consequences of social stratification, as well as the processes that maintain or challenge these inequalities over time.