  A wide variety of cultural practices take the form of "tacit" knowledge,
where the rules and principles are neither obvious to an observer nor known
explicitly by the practitioners. This poses a problem for cultural evolution:
if beginners cannot simply imitate experts, and experts cannot simply say or
demonstrate what they are doing, how can tacit knowledge pass from generation
to generation? We present a domain-general model of "tacit teaching", that
shows how high-fidelity transmission of tacit knowledge is possible. It applies
in cases where the underlying features of the practice are subject to
interacting and competing constraints, as is expected both in embodied and in
social practices. Our model makes predictions for key features of the teaching
process. It predicts a tell-tale distribution of teaching outcomes: some
students will be nearly perfect performers while others receiving the same
instruction will be disastrously bad. This differs from most mainstream
cultural evolution models centered on high-fidelity transmission with minimal
copying errors, which lead to a much narrower distribution where students are
mostly equally mediocre. The model also predicts generic features of the
cultural evolution of tacit knowledge. The evolution of tacit knowledge is
expected to be bursty, with long periods of stability interspersed with brief
periods of dramatic change, and where tacit knowledge, once lost, becomes
essentially impossible to recover.
