There are lots of things in software design and development that are driven, not by the result of formal studies, but by convention and pragmatism. That's why defacto standards exist; they arise out of everyday use, and many people discovering over time what works and what doesn't.
When it comes to choosing between camelCase and hyphenated names this is particularly true, as studies are inconclusive:
The study described in this paper shows that although those without training take longer to recognize identiﬁers in the camel case style, all subjects are more accurate when identifying a camel-cased identiﬁer. In addition, subjects trained to use camel casing take less time to identify a camel-cased identiﬁer than an underscore identiﬁer.
The next step is to consider higher-level tasks in more realistic settings. One task would investigate the impact of camel casing versus underscores when reading blocks of code. For example, subjects might be asked to search for a particular identiﬁer. Another task would ask subjects to read natural language paragraphs modiﬁed to use camel casing or underscores. This would enable more direct comparisons with previous work done in psychology such as Epelboim et al.
An eye-tracking study analyzing the effect of identifier style (camel-case and underscore) on accuracy, time, and visual effort is presented with respect to the task of recognizing a correct identifier, given a phrase. Visual effort is determined using six measures based on eye gaze data namely: fixation counts and durations. Although, no difference was found between identifier styles with respect to accuracy, results indicate a significant improvement in time and lower visual effort with the underscore style. The interaction of Experience with Style indicates that novices benefit twice as much with respect to time, with the underscore style. This implies that with experience or training, the performance difference between styles is reduced. These results add to the findings of Binkley et al.’s study. Future work includes conducting more eyetracking studies (with a larger subset of identifiers and larger subject sample), on reading source code consisting of both identifier styles, in the context of a specific task such as debugging. Another possible direction is to determine if there is an advantage for a programmer to change their current style to what is determined to be a better overall style.
Note: The study referenced here as Binkley et al. is the first study I linked to.