For the most part, though, it appears that the dollar has strengthened for substantive reasons: It has become more attractive than other currencies, on a relative basis, because it’s linked to the American economy, which has been growing more rapidly than those of most other developed countries.

What’s more, the monetary cycle in the United States has diverged from that in many other countries: The Federal Reserve has signaled that it is considering interest rate increases, and it has ended bond purchases — or quantitative easing. That reflects the growth of the American economy, a big contrast with Europe and Japan, which are struggling. Last week, the European Central Bank embarked on a new, 1 trillion euro ($1.16 trillion) campaign of quantitative easing, and it has been further lowering interest rates that are already extraordinarily low and sometimes in negative territory.

In Japan, the central bank has also been aggressively expanding its quantitative easing program and keeping interest rates low.

Furthermore, the dollar has worldwide appeal as a “safe haven,” a destination currency in times of global trouble. That role is likely to become more crucial now that the Swiss franc, another traditional haven, has begun to gyrate wildly in value. That has occurred since Jan. 15, when the Swiss National Bank eliminated the franc’s peg to the euro and started charging banks interest of 0.75 percent to hold their reserves.

The Greek elections on Sunday, which have already unnerved financial markets, could well add more immediate luster to the dollar.

Whether this is truly good for the United States is another matter.

There are some clear benefits, which Mr. Prasad enumerated. Global money flooding into dollar-denominated investments tends to lower interest rates in this country. That typically makes home mortgages cheaper, and a variety of assets more valuable, including houses, bonds and stocks. Americans abroad can buy more with dollars, and imported goods like clothing are cheaper. Add that to the windfall for consumers coming from lower oil prices and it could stimulate the economy.

But there are obvious problems, too. There could be negative spillover effects in emerging markets, where corporate debt has increasingly been dollar-denominated. As that debt becomes more expensive in local currencies, financial stress is likely to heighten.