On a cold day in February 1999, a crowd gathered at Georgetown University to protest the tide of globalization that was reshaping much of the world. Some of their targets, like international trade, might seem familiar today, but the protesters would not have fit in at a Trump rally. They were young and firmly on the left.

Pietra Rivoli, an economist at Georgetown, was watching, and when one woman seized a microphone and, addressing the crowd, asked, accusingly, “Who made your T-shirt?” Professor Rivoli became intrigued. She decided to find out.

She bought $5.99 T-shirt emblazoned with a colorful parrot in Fort Lauderdale, Fla., and followed its path from Texas cotton fields to Chinese sweatshops and eventually the Walgreens bin in Florida where she had purchased it.

In 2005, she published “The Travels of a T-Shirt in the Global Economy.” Few economics books were as compelling and interesting to me as hers. In a review at the time in The New York Times, I wrote that Professor Rivoli had followed her T-shirt “like Melville followed his whale.” Her tilt was pro-trade and put off by the “moral certainty” of protests, which were erupting against the World Trade Organization in coastal cities like Seattle and Boston.

Professor Rivoli understood the protesters’ concerns about human rights, but did not think shutting down trade would help the intended beneficiaries. A quarter-century later, those left-wing protesters are now middle-aged, and the front lines of the anti-trade movement have passed to the Make America Great Again right, a movement with its own moral certainty.

As President Trump moved forcefully against trade, my thoughts turned increasingly to Professor Rivoli. Now a professor emerita still very much engaged in trade issues, she joked that at least, given the attention that Mr. Trump generated, her students no longer treated her lectures on trade as “nap time.”

In fact, they never did. Her book sold 300,000 copies, half in English and half in foreign translations — a remarkable total for a book on trade. We spoke in June and followed up last week, just as Mr. Trump was rolling out a preliminary trade agreement with Vietnam.

With the president seeking more agreements while threatening U.S. trading partners with double-digit tariffs on Aug. 1, Professor Rivoli was as nuanced as ever, still pro-trade, still listening to the other side. Viscerally on guard against journalistic simplification, she cautioned me not to discount those who resented the effects of trade on inequality or mourned a seeming loss of national sovereignty when cheap imports cost jobs at home. Though skeptical of protectionism, she conceded its potential as a negotiating tool even as she questioned America’s approach. We spoke about tariffs, T-shirts and Trump.

This interview has been edited and condensed.

You wrote about anti-W.T.O. protests. Now we’re seeing anti-trade or anti-globalism coming from the populist right. Is this the same phenomenon in different ideological clothes?

I think they’re completely different. Thirty or so years ago, anti-globalization was primarily a youth-activist movement. Concerns were environmental degradation, sweatshop labor, the power of multinational corporations in writing the rules. And if we fast-forward to the present day, many of those demands have been met or addressed.

For example?

We now have environmental protections in all of our trade agreements. And companies pay a lot more attention to worker welfare in their supply chains. But today, on the right, it’s very different. First of all, the Trump tariffs are pretty broadly unpopular. You certainly don’t see any broad-based pro-tariff activism or enthusiasm. You don’t see pro-tariff rallies or protest movements against trade.

That said, I do agree that global elites have shown condescension toward those who have been harmed by trade.

Suppose you believe that on balance, trade is good, and suppose you’re one of the elites. Should you not put that out there?

I think what’s been missing is the “but” that comes after “trade is good.” There are very strong arguments in favor of international trade. But trade also has consequences that are sometimes glossed over. Often there isn’t time. If you only give me two words, I’ll say, “Tariffs bad.”

Why is trade so unpopular?

The classic explanation is that the costs of trade are concentrated and the benefits are diffused. But today, the hot global issue on the right is immigration. That brings people on board with Trump’s trade agenda.

Is trade sort of a scapegoat? U.S. manufacturing fell off the roof way before trade got anywhere near where it is today.

There’s a persistent myth that U.S. manufacturing has collapsed. The value of manufacturing goods that the United States produces has never been higher, and it has grown consistently over time.

Now what has collapsed is manufacturing employment. It has been falling, as a percent of overall employment, since the Second World War. The reasons are, first of all, advances in technology and automation. Each worker now produces much, much more.

A second factor is the kinds of things that we produce in the United States have changed. And this is largely due to trade liberalization. Labor-intensive production has tended to shift to other countries. When China joined the W.T.O., that employment just fell off a cliff. That competition was toughest among blue-collar workers. The result was widening inequality.

A theme in the book was that what we call free trade is often not free and entails assistance, subsidies. Nations cheat on the concept.

Trade is a voluntary exchange. But it has to take place under a set of rules. Rules can be written to my advantage or to your advantage. For many, many years, textile and apparel producers in the United States used their political power to protect themselves from competition. And we have many examples of protected industries. We could talk about tires, solar panels, electric vehicles, steel.

Mr. Trump has said over and over that other countries are ripping us off. Did you find that in T-shirts?

I’m not crazy about the term “ripping us off.” But it is true that the United States has had very low barriers to trade compared to many other countries. So I actually think President Trump is correct to point out that it has been easier for other countries to sell to the United States than the reverse.

And I think most economists would agree that negotiating for better market access makes sense. The reason we have Japanese auto manufacturing plants in the United States is because of the actual or the threat of trade barriers, quotas, tariffs and so forth. Ronald Reagan scared the Japanese manufacturers.

Are you saying the threat of tariffs can be effective?

My point is that tariffs can be evaluated as economic policy, or as a negotiating tool. But I am not a fan of this hardball approach.

Has Mr. Trump used such threats effectively?

We will see very soon. In the preliminary deal with Vietnam, the United States secured market access improvements. We don’t yet know what the final deal will look like. But even if there is a win-win outcome, I just don’t know the long-run effects of this aggressive approach on our diplomatic relationships and reputation.

What’s in store for the T-shirt consumer?

One thing has changed since I wrote the book: Now the overwhelming majority of our T-shirts come into the United States from Central America, because of the Central American Free Trade Agreement, implemented under President Obama. It allowed apparel from those countries to enter the United States duty-free.

President Trump’s reciprocal tariff plan eliminates that duty-free access. Instead, it puts in place tariffs of 10 percent on most of those Central American countries, higher in Nicaragua and Haiti.

Will tariffs bring T-shirt production home?

I don’t think so. The economics of producing in the United States are just too challenging. But there will be an impact — higher prices and lower consumption.

So there will be some people who will buy fewer T-shirts because of that tariff.

That’s right.

And the T-shirts are going to come from Central America anyway.

Yes. There’s not a lot of positive spillovers from producing T-shirts in more advanced economies.

Howard Lutnick, the commerce secretary, used the example of getting people in the United States to screw in little screws to iPhones.

Right now in the United States, the manufacturing sector is facing severe labor shortages. And those are going to be made worse by the immigration restrictions. So if you are going to build an iPhone factory in the United States, it’s going to be very heavily automated. I think that would be true in T-shirts. We wouldn’t have millions of people sitting at sewing machines.

Where would tariffs be worthy of a second look?

Remember, my first words are always going to be “tariffs bad.” But if I got to pick what kind of production was reshored to the United States I would look for things with profitable advanced technology, with intellectual spillover. Things like pharma and computer chips.

The kind of industries that we predominate in now.

That’s right. The kind of industries that are consistent with our comparative advantage. I don’t think higher taxes on clothing imports are a good idea, in particular, because that tariff is regressive. It hits lower incomes.

You present a strong case for trade, but you’re empathetic to the other side. Are you saying there are costs as well as benefits?

I am saying the consequences of trade are often oversimplified. It is not just about lower prices for consumers versus lost factory jobs. It has negative consequences such as greater inequality and higher risk for firms and communities.

Economists favor more liberal trade because it has positive effects on consumption and economic growth. But economists tend to only have that one yardstick. Voters, citizens, have other interests besides cheap consumer goods. There are voters who care about fairness, about sovereignty. I think that’s a big part of what happened with Brexit.

I don’t think Brexit could pass today. Do you?

Probably not. But it did throw into relief reasons that people don’t jump enthusiastically on the trade bandwagon.

Aren’t voters expressing themselves as citizens when they go into stores?

Purchasing decisions are votes in the marketplace. But it doesn’t mean that trade doesn’t have some of these other consequences.

There have been so many examples of countries that have prospered through trade. Look at all the wealth that has been created in New York. It’s not an accident that New York is a port. Are there counterexamples?

Widespread adoption of liberal trade policies is a postwar phenomenon. U.S. industry grew up behind a very high tariff wall. Britain was very protected by high tariff walls. More recently, the so-called Asian miracle states, China, followed a model of export-led development. I wouldn’t call any of those free trade countries. That said, the postwar era of gradually liberalizing trade has given rise to the highest standard of living we’ve ever seen in the world.

In particular the developing world?

Yes, absolutely. We opened our markets to their goods and services. All of that created a huge rising tide.

So is importing T-shirts a massive global anti-poverty program?

What we found is that countries that start to specialize in developing products and services to sell to countries like the United States — that is an effective path out of poverty.

Does the United States have an economic interest in relieving poverty? Or is it zero-sum — the poorer they are, the richer we are?

Adam Smith’s fundamental insight is that trade is not a zero-sum game. We have hardly any barriers among the 50 states. We don’t think “I win/you lose” when we trade between Indiana and Kentucky.

In the book, you halfway joked that we’re never sure whether it’s the best T-shirts that make it into the country or just the best negotiators.

Right now negotiation is the name of the game. President Trump has made it clear that he wants countries coming to the negotiating table.

When I wrote the book, we had a world regime under the W.T.O. in which a lot of trade matters were settled. Now we have this different dynamic of a hundred different countries knocking on the door of the White House. My own bias is I think it’s much better to have a single set of rules for everybody.

Does it open the door to potential corruption? “I’ll give you a jet. I’ll give you … something.”

Absolutely. Trading favors is definitely part of the climate.

You said in the second edition of your book that the case for free trade was as strong as ever. Do you still feel that?

I’m a believer in all forms of openness. Think openness to trade and goods, openness to the flow of ideas, openness to investment flows, openness for human beings to travel —

Do you mean migration?

I think in general, mobility is a very positive thing. My father was an immigrant from rural Sicily, a town called Alcamo. His family built wine barrels, by hand.

If we allow the free flow of goods and services and people, that doesn’t just create wealth. It creates a free flow of ideas; it enhances mutual understanding. Right now the trend is to be closing a lot of these doors. Not just trade. We’re closing access of international students. We’re closing immigration. And you know, I am concerned about that.

When “T-Shirt” was published, free trade was pretty much consensus. Did you ever think that tariffs would be front-page news?

I was as surprised as everyone else.