"Coasean" or "Coasian"?

A long-standing, crucial debate in law & economics is the accepted spelling when Ronald Coase's name is converted into an adjective (e.g., "Coasean bargain" or "Coasian bribe"). In case you were wondering, the latest tally in Westlaw's JLR reports:

"Coasean": 812 article references

"Coasian": 566 article references

This ratio of approximately 60%-40% appears to a basically stable equilibrium over time. In the past three years (since 2002), the tally is "Coasean" 187 and "Coasian" 111.

Both terms appear in 53 article references, usually (but not always, surprisingly enough) because the author uses one spelling but cites to an article using the alternative spelling.