How is Among the Hidden an example of science fiction?

Among the Hidden, written by Margaret Peterson Haddix, is considered a science fiction novel. Science fiction is often about futuristic technology, societies, space travel, or life on other planets. This novel is considered science fiction because it is a novel based on imaginary social changes that impact the world.
In this novel, members of society are only allowed to have two children due to an issue with overpopulation. There are members of a Population Police that monitor that each family only has two children. The Garner family however, is breaking this rule and has a secret third child named Luke. Luke discovers that his neighbors also have a secret third child and finds a way to meet this mystery person. The two form a tight bond and form a rebellion against the government.
This novel also includes the thought of growing food indoors and genetically modifying this food.
This novel, like many science fiction novels, warns readers of the dangers of things that are actually happening in the world today.

Science fiction projects society into a future world that is plausible and rational. Science fiction is not fantasy (though it can have elements of fantasy) because it primarily makes rational sense; it is not a world of fairies, hobgoblins, and supernatural creatures who defy the laws of nature. But science fiction does depict a world that is different from our own.
Among the Hidden qualifies as science fiction because it takes place in a future world in which the government—because of food shortages—has strictly limited the number of children a couple can have to two. Population control is not part of U.S. society, but it has happened in other countries, and we can imagine a world in which it would happen again. Science fiction elements include the existence of hidden children who are not "allowed" to exist because they are outside the population limits and a Population Police force, which uses intrusive technology. As the book states:

the Population Police had ways of finding out everything, how they'd do anything to enforce the law.