Though the phrase more so is conventionally spelled as two words, the one-word moreso gained ground in the late 20th century and continues to appear despite the disapproval of usage authorities and of spell check. Among the major dictionaries, only the Oxford English Dictionary lists the one-word form, and even the OED calls it a “chiefly U.S.” variant of the two-word form.
More so strictly means that to a greater degree, and so recalls an adjective or adverb used earlier. For example, in the sentence, “Gina is studious, and Eleanor is more so,” so recalls the adjective studious, and so is a crucial element of the sentence while more is not. We could change more to less or equally, for instance, and the sentence would still make sense. So because more and so function separately, changing more so to moreso in cases like this is difficult to justify.
But more so is used in several other ways, some of which are not strictly logical. Consider this example:
It’s not that Badet wasn’t happy for his peers. More so, he couldn’t understand why he was being overlooked. [Orlando Sentinel]
Here, so does not recall an earlier word, and more and so do not perform separate functions. They work together as an adverbial phrase similar to even more or rather. If we accept this use of more so—and we have no choice, because it’s common—there’s no reason we shouldn’t accept the compound moreso in its place. In fact, accepting moreso as an adverb synonymous with even more or rather takes care of any quibbles we might have over so’s lack of antecedent.
No matter what we say here, moreso is not fully accepted and is considered wrong by many in the business of making such judgments, so the word is probably best avoided in school papers and other types of formal writing. Dictionaries will probably add it eventually, but that doesn’t mean your teacher or professor won’t lower your grade if you use it. No one questions more so, so it’s always the safer choice.
Also, when using either more so or moreso, make sure that more would not work in its place (see the final group of examples below).
Though an ngram graphing the use of moreso and more so over the last century barely registers moreso, on the web the one-word form appears about once for every 12 instances of more so. The margin is wider—about 25 to one—in Google News searches covering the last few years. When we limit the Google News search to American and Canadian news websites, the ratio is 20 to one. It’s about 300 to one on British news websites, and Australian and New Zealand publications are in the middle, at roughly 40 to one. So moreso does appear to be primarily North American, and it’s especially common in the sorts of writing that fill the bulk of Google’s indexed sites.
Moreso is often used where so has an antecedent (i.e., an earlier word that it recalls). In such cases, the two-word spelling would probably work better—for example:
So having a player’s history of head injuries is just as important as his history with knee injuries and perhaps even moreso. [ESPN’s NFL Nation blog]
That in itself is noteworthy, but moreso is that this mine is located inside Loch Lomond National Park. [PRI (link now dead)]
The album is still often offensively nihilistic, almost moreso because its more believable than the pure, captivating malevolence … the group started out with. [Capital New York]
Sometimes the so in more so/moreso has no antecedent, so the one-word form is less questionable—for example:
[W]edding season can really be a buzzkill to your already busy summer. But moreso, it can be a burden to your wallet. [Chicago Now]
First, Forte is franchised and clearly not happy about it. Moreso, he’s unhappy that the Bears just signed a pretty penny to get Michael Bush. [PatsPulpit]
And some writers unnecessarily use moreso in place of the briefer more:
Even moreso than Ellsworth, though, Donnelly has struggled to raise money early in his campaign. [Washington Post’s The Fix blog]
Even moreso than Burns, Xerox shareholders had a down 2011 with the company’s stock price finishing the year at $7.96. [Rochester Democrat and Chronicle]
Baseball is supposed to be fun, and no movie epitomizes this moreso than the Sandlot. [Arizona Sports]