Higher-order process calculi are calculi in which processes can be communicated. We study the expressiveness of strictly higher-order process calculi, and focus on two issues well-understood for first-order calculi but not in the higher-order setting: synchronous vs. asynchronous communication and polyadic vs. monadic communication. First, and similarly to the first-order setting, synchronous process-passing is shown to be encodable into asynchronous process-passing. Then, the absence of name-passing is shown to induce a hierarchy of higher-order process calculi based on the arity of polyadic communication, thus revealing a striking point of contrast with respect to first-order calculi. Finally, the passing of abstractions (i.e., functions from processes to processes) is shown to be more expressive than process-passing alone.
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