These findings are all based on what voters report in surveys. The same is true if partisanship is measured in a subtle, implicit way. Here is the conclusion of a notable study:
Reporting an Independent political identity does not guarantee the absence of partisanship. Independents demonstrated considerable variability in relative identification with Republicans versus Democrats as measured by an Implicit Association Test…To test whether this variation predicted political judgment, participants read a newspaper article describing two competing welfare or special education policies. The authors manipulated which policy was proposed by which party…Regardless of the policy details, these implicit partisans preferred the policy proposed by “their” party, and this effect occurred more strongly for implicit than explicit plan preference. The authors suggest that implicitly partisan Independents may consciously override some partisan influence when making explicit political judgments, and Independents may identify as such to appear objective even when they are not.
A second key point: In many other respects, voters are not “declaring independence” from political parties. In fact, the American electorate is much more partisan than in the recent past. Consider these points:
1) The number of “pure” independents is declining. It was nearly twice as high in the early 1970s as now.
4) Partisans report less favorable feelings toward the opposite party and express more distress at the thought of their son or daughter marrying someone of the opposite party.
None of this is surprising, given the well-documented polarization among Democratic and Republican party leaders.