A roadmap of when to use ‘co-‘ in writing

I’m not holding my breath about world peace, and I’m not any more sanguine about consensus on the prefix co-, but here’s an appeal about coming to terms about this term.

Generally, no hyphen is required to link this prefix to root words. In the case of many prefixes, the element is initially appended (with a hyphen) to the root word, but as readers and writers become accustomed to the new construction, the linking device is omitted, and the components are joined.

However, some constructions resist this transformation because the resulting fusion looks odd. We make exceptions for certain contiguous vowels ( anti-inflammatory) or for some awkward-looking combinations (pro-choice, though proactive and most other pro- constructions are closed). The default setting, at least in American English, is to discard the hyphen and close up the resulting space.