It is well known that contract incompleteness can arise from the impossibility of planning for all
future contingencies in a relationship (e.g. Williamson (1975)). In this paper it is shown that whether
or not such incompleteness constrains the effiffifficiency of the contract is very sensitive to assumptions
concerning the timing of the resolution of uncertainty. It is shown that when agents must respond to an
unforeseen contingency before being able to renegotiate the contract, then contract complexity is a
binding constraint, a case that is called ex post hold-up. Secondly, it is suggested that the amount of
multi-tasking can provide a measure of contract complexity. When complexity is low, contingent
contracting is effiffifficient, while subjective performance evaluation is more effiffifficient when complexity is
high. In this case the optimal contract for ex post hold-up is based upon the ability of humans to make
subjective judgements that are in some cases more informative than explicit performance measures.
Moreover, the effiffifficiency of the contract is not sensitive to human error per se, but is an increasing
function of the correlation in judgements between the contracting parties.