Why was the Three-Fifths Compromise necessary? Select all that apply. Northerners feared that Southerners would manumit their slaves, causing a mass migration to northern states. Northerners believed that if slaves were counted as part of the population, then the Southern states would dominate the House of Representatives. Southerners feared that they would be taxed twice if each slave were counted as property as well as part of the population. Southerners were concerned that Northerners would institute plantations and drive down the price of their cash crops.