Pairs trading is a strategy where you buy one stock and sell another. This strategy can make money regardless of whether the overall market is going up, down, or staying the same. Researchers looked at whether this strategy works in countries outside the US and during different market conditions. They found that in most countries, pairs trading can make a profit, even during bear markets. The strategy still works well and can make money even after accounting for transaction costs. It also helps diversify investments by not being closely tied to stock market performance.