This project contains the files for the ideology and invariance registered report project. The Stage 1 manuscript with in principle acceptance is [here][1] Preregistered code is [here][2] Final code used is [here][3] Additional code for extracting parameters is [here][4] **Abstract for Stage 2 version of paper** People with different ideological identities differ in their values, personality, affect, and psychological motivations. These differences are observed on measures of practical and clinical importance and these differences are the central node tying together theories about the psychology of political ideology; however, they rest on a critical untested assumption: The measures are invariant across ideological groups. Here, we test this assumption across 28 constructs in data from the United States and the Netherlands. Measures are not invariant across ideological divisions; however, estimates of ideological similarities and differences are similar before and after correcting for measurement non-invariance. This gives us increased confidence in the results from this research area, while simultaneously highlighting items that are not comparable across political groups. [1]: https://osf.io/r7ekd/ [2]: https://osf.io/udg8e/ [3]: https://osf.io/zjv8e/ [4]: https://osf.io/rpfbv/ [5]: https://osf.io/udg8e/