We measure a specific form of other-regarding behavior, costly cooperation with an anonymous other, among 645 subjects at a trucker training program in the Midwestern US. Using subjects' second-mover strategy in a sequential form of the Prisoners' Dilemma, we categorize subjects as: Free Rider, Conditional Cooperator, and Unconditional Cooperator. We observe the subjects on the job for up to two years afterwards in two naturally-occurring choices – whether to send two types of satellite uplink messages from their trucks. The first identifies trailers requiring repair, which benefits fellow drivers, while the second benefits the experimenters by giving them some follow-up data. Because of the specific nature of the technology and job conditions (which we carefully review) each of these otherwise situationally similar field decisions represents an act of costly cooperation towards an anonymous other in a setting that does not admit of repeated-game or reputation-effect explanations. We find that individual differences in costly cooperation observed in the lab do predict individual differences in the field in the first choice but not the second. We suggest that this difference is linked to the difference in the social identities of the beneficiaries (fellow drivers versus experimenters), and we conjecture that whether or not individual variations in pro-sociality generalize across settings (whether in the lab or field) may depend in part on this specific contextual factor: whether the social identities, and the relevant prescriptions (or norms) linked to them that are salient for subjects (as in Akerlof and Kranton (2000); (2010)), are appropriately parallel.