What other kind of system of social stratification exists aside from slavery, caste, and class?

Slavery, though it still exists, is no longer legally sanctioned. However, when we consider who is currently enslaved, in many instances, we find women and girls from impoverished backgrounds. This leads me to argue that you are overlooking one key and persistent form of social stratification: divisions which exist according to sex or gender.

Sexism, or systemic discrimination against women, exists in every society due to lingering stereotypes and prejudices that have persisted for centuries. The practice of turning women and girls into sex slaves is undoubtedly drawn from the notion that female bodies exist for male consumption and, therefore, can be commodified.

However, sexism manifests differently in every society. In the United States, for example, it is discussed when women talk about reproductive freedom and pay equity. In Saudi Arabia, it might surface in a discussion about exercises of female autonomy, such as the privilege to drive. Throughout the world, women are concerned with violence directed against them (e.g., rape, domestic violence, sexual abuse), and the limitations that might be imposed upon their personal and professional lives due to sex discrimination.

It is no coincidence that men control three-quarters of the world's wealth and hold most positions of political power, globally. Still, not all forms of sex discrimination are created equally.

Law professor Kimberle Crenshaw coined the term "intersectionality" to denote the ways in which oppressive institutions or ideologies are interconnected. For example, the average salary for a white woman in the United States is seventy-seven cents for every dollar that a white man makes. However, a black woman's average salary is sixty-four cents for every dollar that a white man makes. Because a white woman benefits from race privilege, she has more earning power and more opportunities for promotion than a black woman who may have the same qualifications. Thus, according to Crenshaw, it is not really possible to examine gender stratification without also looking at racial stratification.