In a beautifully written and insightful paper just published in Cognition, a team of researchers from Duke reports a series of studies on the “ought” implies “can” principle in ordinary moral cognition.* The main question was whether there is a conceptual entailment from what someone “ought” to do to what they “can” do, as those concepts are ordinarily understood.
In one experiment, the team tested people’s judgments about two versions of a case (within-subjects). Both versions of the case started the same way:
Adams promises to meet his friend Brown for lunch at noon today. It takes Adams thirty minutes to drive from his house to the place where they plan to eat lunch together.
The “low blame” version of the case ended like this:
Adams leaves his house at eleven thirty. However, fifteen minutes after leaving, Adams car breaks down unexpectedly. Because his car is not working at that time, Adams cannot meet his friend Brown at noon, as he promised.
The “high blame” version of the case ended this way:
Adams decides that he does not want to have lunch with Brown after all, so he stays at his house until eleven forty-five. Because of where he is at that time, Adams cannot meet his friend Brown at noon, as he promised.
Participants then rated whether “At eleven forty-five, it is still true that Adams ought to meet Brown at noon.” People tended to agree with this “ought” attribution in the high blame version, while they tended to disagree in the low blame version. (The order in which participants read the two scenarios didn't affect their "ought" attributions.)
In a follow-up study, the research team collected judgments about “ought,” “can,” and “blame.” They found that “ought” and “blame” judgments correlated, that “can” and “blame” judgments correlated, but, critically, that “ought” and “can” judgments did not correlate.
Based on these findings, the research team concludes that there is no conceptual entailment from “ought” to “can.” They also suggest that philosophers who claim otherwise might be “distorting their judgments” about what someone ought to do based on a “motivation to withhold blame.”
I encourage you to read the whole paper. At only 6 pages long, it's a great return on the investment of time!
*Chituc, V., Henne, P., Sinnott-Armstrong, W., & De Brigard, F. (2016). Blame, not ability, impacts moral 'ought' judgments for impossible actions: Toward an empirical refutation of 'ought' implies ‘can.’ Cognition, 150(C), 20–25. http://doi.org/10.1016/j.cognition.2016.01.013