Present bias and collective dynamic choice in the lab?

We study collective decisions by time-discounting individuals choosing a common consumption stream. We show that with any heterogeneity in time preferences, utilitarian aggregation necessitates a present bias. In lab experiments three quarters of "social planners" exhibited present biases, and less than two percent were time consistent. Roughly a third of subjects acted as if they were pure utilitarians, and the rest chose as if they also had varying degrees of distributional concerns.