Abstract. This article analyzes the extent to which the format and the “Do not know” (DK) protocol of political knowledge questions influence the size of the gender gap. By using a set of experiments that manipulated the DK protocol and the format (open vs. closed ended) of political knowledge questions in a face-to-face representative survey of the Spanish population, we show that the format of the questions (open vs. closed ended) is not relevant in explaining the gender gap. DK protocols, however, influence outspoken levels of political knowledge differently for men and women. DK-discouraging protocols tend to encourage the emergence of hidden knowledge among women, whereas they mostly boost guessing among men. This finding suggests that the meaning and the use of the DK option appear to be gender biased.