As agricultural capacity increases, city populations tend to swell in size, eventually filling up the settled land around the city center. In ancient times, large cities suffered from terrible hygiene problems. The introduction of aqueducts and sewage systems helped to contain the spread of disease and allow city populations to grow into the millions. In the modern era, the problems of overcrowding are often psychological, as many urban dwellers have trouble coping with the constant pressure of bustling humanity. As unhappiness increases, the people learn to escape through movies and computer games.