The relative importance of ideological orientations and material self-interest as determinants of political attitudes is still discussed. Using a novel list experiment on opposition to progressive taxation embedded in a large representative Danish online survey (N = 2,010), I study how social desirability concerns bias the conclusions survey researchers draw regarding the influence of self-interest (gauged by income) and ideology (measured by left-right self-identifications) on public opinion. I find that right-wingers are much less opposed to progressive taxation when attitudes are measured indirectly and unobtrusively by means of the list experiment relative to asking directly about their opinions. In fact, rightists are no more against progressive taxation than leftists and centrists. Furthermore, opposition to tax progressivity is considerably lower among low-income individuals when social desirability bias is addressed, thereby increasing the attitudinal gap between low- and high-income individuals. The implications of the findings are that survey research risks exaggerating the importance of ideological orientations and underestimating how much political views reflect material self-interest.