Slavery is fundamentally an economic phenomenon; it occurs when the net benefits to free society are positive. From antiquity until the mid-twentieth century, slavery was primarily a profitable way of organizing the labor force, at least to those other than the slaves themselves. Slave societies passed laws to perpetuate the peculiar institution, often acknowledging both the property value and the personhood of slaves. Much scholarly research focuses on the enslavement of Africans on several continents starting in the mid-1400s, the subsequent four centuries of the transatlantic slave trade, and the plantation economies of the New World. But slavery has thrived in many parts of the world for most of human history, with slaves representing productive investments and tradable wealth for their owners. More recent forms of slavery depart from this pecuniary model—enslaved political enemies may not yield monetary rewards but instead serve as symbols of dominance for tyrannical governments. Nonetheless, slavery and the legal and political institutions that support it have largely been driven by economic factors.