Recently, UCLA Law School Professor Stephen Bainbridge wrote about his allegiance to writing "decision making" as two words. In support, he cites the Chicago Manual of Style. I think that the Manual actually makes a distinction based on usage. When the compound is used as a noun, the two words are separate (e.g., "future decision making by lenders and homeowners" Bank of America, NA v. Roberts, 217 Cal. App. 4th 1386, 1394 (2013)) but when used as an adjective, the hyphenated form is used (e.g., "the decision-making procedure" In re Vicks, 56 Cal. 4th 274, 310 (2013)). I agree that the Manual doesn't support the use of "decisionmaking" as a single compound word.
A quick perusal of California case law reveals a not unexpected lack of consistency. This may be in part the fault of legal publishers because I've noted at least one instance in which different publishers used different forms. The California legislature, however, clearly parts company with the Manual. I found 195 examples of statutes using "decisionmaking" as a single compound word, regardless of usage. For example, Education Code Section 51220(b)(5) refers to "decisionmaking skills", an adjectival use, while Education Code 49341(b) refers to "informed judgment, decisionmaking, and operating procedures", a nominal use. I found only one statute that uses "decision-making", Health & Safety Code Section 57008(b)(1), and that was in reference to the title of a technical report. I'm relying on the versions of the California Codes published on the California Legislature's website. It is possible that other publishers of the California Codes use different spelling conventions.
A Gerund Grinding Gloss On Gerunds
However spelt, "decision making" consists of a noun ("decision") and a gerund ("making"). A gerund is the form that a verb takes when used as a noun. For example, 11 U.S.C. § 522(f)(1) provides that "the debtor may avoid the fixing of a lien". In that phrase, the gerund form of the verb "fix" is used as a noun (the direct object of "avoid"). See Farrey v. Sanderfoot, 500 U.S. 291, 296 (1991).