The Japanese yen has strengthened against the U.S. dollar over the past 20 years. This is due to several factors, including lower interest rates in Japan compared to other countries, a large aging population, and Japan running a trade surplus where it exports more than it imports. When demand for the yen is high due to exports and investments into Japan, it causes the yen to strengthen as there is more demand than supply for the currency. Conversely, when Japan runs a trade deficit or its investments abroad increase the yen supply, it causes the currency to weaken. Due to Japan's ongoing trade surplus, this has kept the yen strong compared to the U.S. dollar.