Kingsley Davis and Wilbert Moore, in their 1945 article 'Some Principles of Stratification,' presented a functionalist theory arguing that social stratification is not only universal but also a functional necessity for any complex society. Their central argument posits that all societies must ensure that the most important and functionally necessary positions are filled by the most qualified and capable individuals. To achieve this, societies develop a system of differential rewards and prestige.

According to Davis and Moore, positions vary in their functional importance and require different levels of skill, training, and talent. For instance, a surgeon's role is considered more functionally important and requires more extensive training than a janitor's. To motivate individuals to undergo the necessary training and perform these demanding roles, society attaches greater rewards (e.g., higher income, prestige, power) to these positions. This differential reward system acts as an incentive, ensuring that a sufficient number of talented individuals are attracted to and adequately perform these crucial roles.

They also described stratification as an 'unconscious device' because it is not typically the result of a deliberate, conscious conspiracy or plan by a ruling elite. Instead, it emerges organically as societies develop mechanisms to allocate personnel to various roles. It's an institutionalized system that evolves to meet the functional prerequisites of society, ensuring its smooth operation and survival. While individuals may consciously strive for higher positions, the overall system of stratification operates without a collective, explicit design. Critics, however, argue that this theory justifies existing inequalities and overlooks power dynamics, inherited advantages, and the potential for talent to be stifled by lack of opportunity.