Federal tax revenue plunged $138 billion, or 34%, in April vs. a year ago — the biggest April drop since 1981, a study released Tuesday by the American Institute for Economic Research says. When the economy slumps, so does tax revenue, and this recession has been no different, says Kerry Lynch, senior fellow at the AIER and author of the study. "It illustrates how severe the recession has been." For example, 6 million people lost jobs in the 12 months ended in April — and that means far fewer dollars from income taxes. Income tax revenue dropped 44% from a year ago.