Later
Here's a study comparing Sweden's compliance rate with that of Italy's, and finding them roughly similar:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4823977/
It's not just about filing returns, but about the percentage of each person's earned income that is properly accounted for on those returns.
Looking at the system at that level will not be 100% but then the numbers are rounded anyway. The question I ask in the context of this discussion is if the 96% as estimated by the Swedish Tax Agency, is bigger or smaller than the 83.1% provided by IRS, knowing that both number include a certain amount of uncertainty. Both numbers are provided by the state agency of each country so some assumption of good-faith and competency should be applied.
tptacek's description of the paper he cites, i.e. "Here's a study comparing Sweden's compliance rate with that of Italy's, and finding them roughly similar" leaves out the crucial point that the whole paper is about a game, not real taxes.