The effects of the Atlantic slave trade on West African societies were complex and characterized by change over time. At first, the slave trade was actually controlled by West African kings, who offered enslaved people instead of the gold that many early European voyagers sought to trade for. Over time, as the kingdoms of Europe established colonies overseas, the increased demand for labor that accompanied this transformation altered the trade. The slave trade enriched the kings of such empires as the Kongo, but it did so at a massive cost. It led to endemic warfare in the region, as Europeans sought captives to enslave. It later led to major social stresses, as village life was often disrupted by slave gangs who kidnapped young people for sale. Over time, European powers sought to conduct the trade on their terms, building large fortresses that served as the centers of slave trade and the ports of departure for millions of unfortunate people. One very significant way that the trade affected Africans was by causing a major gender imbalance in African towns and villages. Since slavers favored male captives, fewer were left behind. This led to polygamous marriage practices, one significant way in which traditional African lifeways were disrupted. The slave trade also deprived West African kingdoms of healthy young laborers, which not only ripped families and villages apart, but also inhibited the development of the region's economy.