Does a treaty reduce the tax my home country withholds on dividends I receive there?

Often yes. Income tax treaties commonly cap the rate at which the source country can withhold tax on dividends, interest, and royalties paid to a resident of the other country, frequently reducing a high domestic withholding rate to something like 15% or lower on dividends. To claim the reduced rate you usually give the paying institution a certification of your U.S. residency, sometimes supported by IRS Form 6166, a U.S. residency certification you request from the IRS. Any foreign tax that is properly withheld under the treaty can then be claimed as a Foreign Tax Credit on your U.S. return to avoid double taxation. Rates and procedures vary by country and by the specific treaty article, so check the relevant treaty and your foreign payer's requirements. Do not pay more foreign withholding than the treaty allows.