What's really behind negative bond yields?

The fact that short-dated bond yields are negative in many European countries is often attributed to the prospect of substantial purchases of euro-zone government debt by the ECB under the policy of quantitative easing (QE). However, yields are most negative outside the euro-area, in Switzerland and Denmark. We would give at least as much weight to the role of negative official interest rates and the potential for an offsetting gain on the currency.