What was a consequence of people migrating from the rural areas to the city?

There have been several great waves of urbanization and industrialization. In western Europe, Britain, Canada, and the United States, these were typical of the nineteenth century. In much of the rest of the world, urbanization has been increasing in the twentieth and twenty-first centuries. Over the entire world, most countries are still becoming increasingly urbanized.
Urbanization is associated with dramatic industrial growth, GDP increases, cultural vitality, cosmopolitanism, and network effects in technological and other forms of innovation. It is also associated with declining birthrates. Extra children, rather than becoming useful farmhands, are expensive, requiring education for several years before they become economically productive in urban environments.
On the negative side, rapid urbanization can lead to the development of insalubrious slums, replete with epidemic diseases, bad air and water pollution, and other forms of environmental degradation.