Birthright citizenship is a controversial topic because many countries do not recognize it but the U.S. still does. According to the Center for Immigration Studies, only 38 countries have birthright citizenship.
The idea of this is that you gain citizenship by birth when you are born in the country regardless of your parent’s status.
In the U.S.
In the U.S., you can get birthright citizenship in a few ways beyond just being born in the country. You can also gain citizenship this way if you are born anywhere to at least one parent who is a U.S. citizen with permanent residency in the U.S.
There are some variances in the law when it comes to birth outside the U.S. and the residency requirements of your parent who is the U.S. citizen. This also includes birth in U.S. territories.
Another way to gain U.S. citizenship by birth is if you are found on U.S. soil with no known parents. You must be age five or younger at the time authorities find you.
Other countries
Those countries that do not recognize birthright citizenship, will only allow automatic citizenship if one parent is a citizen of the country. Some countries have additional requirements you must meet or may allow you to become a citizen more easily if you did not gain status by birthright.
Controversy
Birthright citizenship is controversial in the U.S. with many people believing that it is a right of people granted under the constitution. Those that oppose it point to the fact that most of our ally countries do not recognize it and that it is an outdated concept that requires modernization.