We investigate factors that affect speech overlaps in conversation, using large corpora of conversational telephone speech. We analyzed two types of speech overlaps: 1. One side takes over the turn before the other side finishes (Turn-taking type); 2. One side speaks in the middle of the other sides turn (Backchannel type). We found that Japanese conversations have more short Turn-taking type of overlap segments than the other languages. In general, females make more speech overlaps of both types than males, both males and females make more overlaps when talking to females than talking to males. People make less overlaps when talking with strangers than talking with familiars, and the frequency of speech overlaps is significantly affected by conversation topics. Finally, the two conversation sides are highly correlated on their frequencies of using Turn-taking type of overlaps but not Backchannel type.