Teasdale and Owen (2005) examined the results of IQ tests given to Denmark male conscripts. Between 1959 and 1979 the gains were 3 points per decade. Between 1979 and 1989 the increase approached 2 IQ points. Between 1989 and 1998 the gain was about 1.3 points. Between 1998 and 2004 IQ declined by about the same amount as it gained between 1989 and 1998. They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16-18-year-olds." The same authors in a more comprehensive 2008 study, again on Danish male conscripts, found that there was a 1.5-point increase between 1988 and 1998, but a 1.5-point decrease between 1998 and 2003/2004. A possible contributing factor to the more recent decline may be changes in the Danish educational system. Another may be the rising proportion of immigrants or their immediate descendants in Denmark. This is supported by data on Danish draftees where first or second generation immigrants with Danish nationality score below average.
Question: How many years was the gains 3 points per decade for male IQ tests?
Answer: 20