We report from a large-scale randomized field experiment conducted on a unique sample of more than 15,000 taxpayers in Norway, who were likely to have misreported their foreign income. We find that the inclusion of a moral appeal or a sentence that increases the perceived probability of detection in a letter from the tax authorities almost doubled the average self-reported foreign income. The moral letter mainly works on the intensive margin, while the detection letter mainly works on the extensive margin. We also show that the detection letter has large long-term effects on tax compliance.