Donald Trump’s attitude toward immigrants since the presidential campaign has not been politically correct. Just think of the idea of building a wall, which already partially divides the U.S. and Mexico, or the way he has occasionally referred to the Mexicans who want to enter the United States.

These episodes came to mind in a recent television interview when Trump announced his intention to change the rules of citizenship, a plan that many consider impossible, while others accept as legitimate. It is a discrepancy that demands clarification.

The rules that determine citizenship are different from country to country. There are countries that adopt the "jus soli" criteria, which means that when a child is born, it automatically receives the citizenship of the country where the birth took place, unless the parents are opposed. On the other hand, there are countries that prefer the standard of "jus sanguinis," which means that the child receives the citizenship of its parents.*

As history shows, countries can change their position whenever they feel the need to do so. In other words, when a country needs to increase the birth rate, jus solis will be the preferred standard, but when there are no demographic problems, countries tend to adopt the jus sanguinis standard. However, this rule is not absolute since it is affected by political, economic and cultural factors. That explains why some, like Marine Le Pen, are against citizenship as an automatic right.

Going back to Trump’s plan, the president is not finding it easy to change the existing rules through an executive order. The subject is too complex, and appears to be unavailable based on the whims of a single president, whoever he might be.

In fact, U.S. citizenship as an automatic birthright for those born in the U.S. is a rule that derives from the 14th Amendment. Perhaps it is worth remembering that the U.S. Constitution was written by the Founding Fathers and that it is necessary to preserve its initial spirit, despite the fact that circumstances have required changes over time.

For that reason, to change the rules of citizenship, a Constitutional amendment would be necessary, and that is beyond the president’s powers.

There is, however, an aspect of the law that may favor Trump in this matter. In fact, the first section of the 14th Amendment recognizes the right to U.S. citizenship for those who are born or naturalized in the U.S., but it also says they must be “subject to the jurisdiction" of the United States.

As we can see, the Constitution does not make any reference to the parents of newborn or naturalized children. This is an important detail that may lead to a legal battle if Trump thinks that the parent’s legal condition should be directly applied to their children. It could result in a decision that populists would favor, even if the demographic data are not favorable.

In fact, demographics show that in order to maintain or increase the population, the fertility rate – the number of children a woman of fertile age should have – is 2.1. However, in the United States and in developed countries in general, fertility is falling and it no longer reaches that number. This is a problem that could soon have a high political cost unless policies are enacted to increase the birth rate.

That doesn’t seem to worry Trump much. A president should not give up on an idea that fits perfectly with his conception of 'the other.' It is an idea supported by the most conservative voices of those who will soon claim that there should be an increase in the domestic birth rate. Judging from this retrograde point of view, the only regret will be that the miscegenation that took place for several generations will not allow the degree of purity that eugenics demand.

We’ve come a long way since Alexis de Tocqueville wrote “Democracy in America.”

*Editor's note: Jus soli is a Latin phrase meaning "right of the soil," commonly referred to as birthright citizenship in the United States, and is the right of anyone born in the territory of a state to nationality or citizenship. Jus sanguinis is a Latin phrase which provides that citizenship is not determined by place of birth but by having one or both parents who are citizens of the state.