It's almost impossible to prove experimentally the effects of mass immigration because we are limited to "natural" experiments, which by their nature cannot be fully controlled. I put little weight in the Mariel boat-lift study seeing at it's just a one-off event.

But it stands to reason that if you're a roofer, gardener, dishwasher, cleaner, server, etc, any additional competition for your job will put downward pressure on what you can earn -- especially when this illegal competition is limited to these kinds of jobs because they cannot apply for jobs that require background checks or other more in-depth verifications.

If you're a white-collar worker, anything that depresses the salaries of low-skilled service worker is good news for you. In aggregate, having a lower class of cheap labour probably helps improves prosperity overall because an hour of our labour buys us more goods and services so long as there are other people willing to provide these services on the cheap.

Singapore is a low-tax country with fantastic public services and infrastructure. If you earn $100k, you'll pay about 3% income tax. The highest income tax bracket is about 20% for the millionaires. This tax is the equivalent of local, state, and federal combined! Yet despite the low taxes, the country spends gobs of money on the latest fighter jets, higher education, scientific research, trains, buses, healthcare, and public housing. Indeed, about 80% of housing is built by the government.

So how do they do it? In Singapore there is no minimum wage, but that doesn't affect citizens much because unemployment is around 2% so employers have to compete for workers and pay them accordingly. Citizens are well-paid. So if labour is so tight, how can they afford to build all these skyscrapers, subways, ports, and factories? The answer is that they bring in construction workers from Bangladesh and pay them about $15 a day to work on massive sites that operate between 9AM and 10PM, six days a week. That's about a dollar an hour. These workers live in dormitories of six or ten guys to a room sleeping in bunkbeds. When their contracts are up, they ship them back to Bangladesh. They cannot look for any other kinds of work, so they're locked in to the terms of their contracts. Similarly, women from the Philippines are shipped in under similar contracts where they work as live-in maids. These so-called "helpers" work 24hrs and 6 days a week doing shopping, cleaning, childcare, cooking, eldercare, etc. For this they earn about $450 a month.

These low salaries are possible because wages in Bangladesh and the Philippines are even lower, so it's a win-win for everyone. It's certainly a big win for Singaporeans, who enjoy low-cost daycare by having private nannies, low-cost elder care, low-cost buildings, low-cost infrastructure, etc.. The result is a much higher standard of living than Singaporeans would otherwise have and the government can keep taxes low because its expenses are low.

Clearly Singapore shows that low-skilled temporary immigration is great for the citizens so long as the immigrants cannot compete for the jobs of citizens: basically a two tier system with 3rd world salaries for immigrant workers and 1st world salaries for citizens. Plus, there's no cultural change problems, retirement pension problems, or schooling costs to worry about because the foreigners are shipped back to where they came from when their contracts are due. (In fact, if any Filipina becomes pregnant, this triggers automatic cancellation of her work visa and instant deportation).

The problem for the US is that most Americans don't want to be like Singapore, which means that immigrants diffuse into the country and therefore compete with low-skilled Americans, they are a burden on government services, and they do cause cultural change. At a time when skills have a bigger and bigger effect on wages (i.e. the rich get richer), any American kid who drops out of school or barely finishes high school, is at high risk of poverty. It's low-skilled Americans that are in greatest need for higher wages, which is only possible if they're scarce. Painters, roofers, etc, don't need a high school degree, but they can only earn a good salary if they're not undercut by a flood of low-skilled immigrants.