  In other cases it may be unclear whether redistribution occurred in any of these three senses. Take, for example, the question of whether income redistribution occurred in the United States between 1979 and 1987, accepting as true the findings of a U.S. congressional study (released in March, 1989) which claimed that the average family net income of the poorest fifth of the American population declined by over 6 percent from 1979 to 1987, while it rose by over 11 percent for the richest fifth, and that personal income declined by 9.8 percent for the poorest fifth while rising 15.6 percent for the richest  fifth.[6]  With respect to income quintiles, it seems very likely that diachronic income redistribution occurred between 1979 and 1987 — but only very likely, since it is possible in principle to deny that the income shifts among quintiles were caused by the policies adopted by the Reagan Administration or through other social mechanisms. As noted above, the term ‘redistribution’ is usually taken to require some social mechanism that, whether intentionally or not, causes the shift.