Brennan considers the question at length in his book, precisely because of unreasonable restrictions of suffrage in the past. The level of knowledge he is seeking is not high—knowing the distinction between the legislative, executive, and judicial branches, or the outline of how a congressional bill becomes law, fundamental questions of fact about how the current government works rather than contested questions about history. Shockingly, the majority of the eligible voters in all countries surveyed are unable to achieve better than 50% on basic knowledge tests (relative to their own country—it makes no sense to quiz Swedes about Australian parliamentary procedure).
Brennan considers the question at length in his book, precisely because of unreasonable restrictions of suffrage in the past. The level of knowledge he is seeking is not high—knowing the distinction between the legislative, executive, and judicial branches, or the outline of how a congressional bill becomes law, fundamental questions of fact about how the current government works rather than contested questions about history. Shockingly, the majority of the eligible voters in all countries surveyed are unable to achieve better than 50% on basic knowledge tests (relative to their own country—it makes no sense to quiz Swedes about Australian parliamentary procedure).