That objective reference is necessary for formation of reliable beliefs about the external world is almost axiomatic. However, Condorcet (1785) suggested that purely subjective information--if shared and combined via social interaction--is enough for accurate understanding of the external world. We asked if social interaction and objective reference contribute differently to the formation and build-up of collective perceptual beliefs. In three experiments, dyads made individual and collective perceptual decisions in a two-interval, forced-choice, visual search task. In Experiment 1, participants negotiated their collective decisions with each other verbally and received feedback about accuracy at the end of each trial. In Experiment 2, feedback was not given. In Experiment 3, communication was not allowed but feedback was provided. Social interaction (Experiments 1 and 2 vs. 3) resulted in a significant collective benefit in perceptual decisions. When feedback was not available a collective benefit was not initially obtained but emerged through practice to the extent that in the second half of the experiments, collective benefits obtained with (Experiment 1) and without (Experiment 2) feedback were robust and statistically indistinguishable. Taken together, this work demonstrates that social interaction was necessary for build-up of reliable collaborative benefit, whereas objective reference only accelerated the process but--given enough opportunity for practice--was not necessary for building up successful cooperation. (Contains 4 figures.)