One of the most influential and well-known promises that Donald Trump made during his campaign for president was that he would build a "big, beautiful wall" along the southern border between the United States and Mexico. While this idea has been championed by those who claim that America is being "invaded" with undocumented immigrants, history tells a different story. In actuality, making it harder to cross the border increases the immigrant population.

To properly understand how this seemingly contradictory statement can be true, it is imperative to understand one of the most basic concepts of economics: cost. There are two types of cost that go into calculating the total cost of anything; one is the actual price of the product in dollars, and the other is opportunity cost. To put it simply, opportunity cost is what you give up (other than money) when you buy something. The less you have to give up, the more likely a consumer is to buy that product. Trust me, this concept will be useful later.

Before the United States militarized its border, immigration laws were very laxly enforced, making it very easy for people to get across the border from Mexico and come to America for work.

At first glance, you would think that this would dramatically increase the amount of illegal immigrants and migrant workers living in the country, but they were not actually living in the country. Because it was much easier to get from Mexico to America, this also means that it was much easier to get from America to Mexico. Being able to cross the border freely meant that people would go to the United States for a period of several months to work and make money while their families would remain in Mexico. Then, when they've made enough money in the States, they'll come back to Mexico and spend time with their families for the rest of the year, and then the cycle would repeat itself.

However, this changed in 1973 when Leonard Chapman took over as the Commissioner of Immigration and Naturalization in 1973. Chapman was a former general who served in the Vietnam War, which is interesting. Vietnam was essentially a border war, with the South Vietnamese trying to stop North Vietnam from sending in people to disrupt government function. Essentially, the whole objective of this war was to secure the border. Chapman picked up where he left off at his new job, visiting every office in his agency and strictly enforcing current laws while creating new ones.

What this did was change the cost of crossing the border.

Now that the border is riskier to cross, people won't do it as much. This doesn't mean, however, that less people will come in from Mexico. What it does is keep people who cross the border into America from crossing back over. The same number of people are crossing the border, but more people are staying. Overall migration is the same, but net migration, how many come in minus how many leave, has increased dramatically. This also means that when workers don't return, they try to reunite with their families, creating to so-called "chain migration problem", and bringing in the young children that would grow up to be the "dreamers" protected by DACA.

From 1965 to 1985, 85 percent of Mexican immigrants who entered the United States went back to Mexico, making the overall increase relatively low. Additionally, if a migrant entered America in 1985, there was a 50 percent chance that they would return to Mexico. In 2010, that number was zero. If Trump supporters really want all of these undocumented immigrants to go back to Mexico, maybe they should actually let them do that. Building walls keeps things out, but nobody really considers what they keep in.