Friedman and Phelps opposed this idea on theoretical grounds, as they noted that if unemployment were to be permanently lower, some real variable in the economy, like the real wage, would have changed permanently. That this should be the case because inflation was higher appeared to rely on systematic irrationality in the labor market. As Friedman remarked, wage inflation would eventually catch up and leave the real wage, and unemployment, unchanged. Hence, lower unemployment could only be attained as long as wage inflation and inflation expectations lagged behind actual inflation. This was seen to be only a temporary outcome. Eventually, unemployment would return to the rate determined by real factors independent of the inflation rate. According to Friedman and Phelps, the Phillips curve was therefore vertical in the long run, and expansive demand policies would only be a cause of inflation, not a cause of permanently lower unemployment.