I have calculated document similarities using Doc2Vec.docvecs.similarity() in gensim. Now, I would either expect the cosine similarities to lie in the range [0.0, 1.0] if gensim used the absolute value of the cosine as the similarity metric, or roughly half of them to be negative if it does not.
However, what I am seeing is that some similarities are negative, but they are very rare – less than 1% of pairwise similarities in my set of 30000 documents.
Why are almost all of the similarities positive?