The emergence of politically driven divisions in online discussion networks has attracted a wealth of literature, but also one which has thus far been largely limited to single country studies. Hence whilst there is good evidence that these networks do divide and fragment into what are often described as "echo chambers", we know little about the factors which might explain this division or make networks more or less fragmented, as studies have been limited to a small number of political groupings with limited possibilities for systematic comparison. This paper seeks to remedy this deficit, by providing a systematic large scale study of fragmentation on Twitter which considers discussion networks surrounding 90 different political parties in 23 different countries. It shows that political party groupings which are further apart in ideological terms interact less, and that individuals and parties which sit at the extreme ends of the ideological scale are particularly likely to form echo chambers. Indeed, exchanges between centrist parties who sit on different sides of the left-right divide are more likely than communication between centrist and extremist parties who are, notionally, from the same ideological wing. In light of the results, theory about exposure to different ideological viewpoints online is discussed and enhanced.