Most English have adjectives have comparative (-er or more/less) and superlative (-est or most/least) forms. The three major irregular adjectives are good-better-best, bad-worse-worst and far-further-furthest. One student wrote farer and farest. I said ‘Those are clear and fit the pattern, but we’ve got these special words further and further’. No-one wrote or said gooder, goodest, badder or baddest. I commented that those are clear and fit the pattern as well, but badder and baddest sound slightly better than gooder and goodest. Jim Croce calls Leroy Brown ‘The baddest man in the whole damned town / Badder than old King Kong’, not ‘The worst man … Worse than King King’. The spell-checker in Pages for Mac accepts badder and baddest, but not gooder and goodest. (It also accepts farer. I assume that’s related to fare (farer – ?a paying customer/traveller) not far. Compare wayfarer. The spell-checker in WordPress accepts badder and baddest, but not gooder, goodest or farer.) (Possibly, the regular adjective forms of far should be farrer and farrest, but it’s not necessary to decide.)
Google Ngrams shows a fascinating result: the usage of gooder has skyrocketed since 1940. Why on earth? It’s certainly related to the usage of do-gooder, which is not a comparative adjective, but a noun. (Maybe it should be good-doer, but no-one actually says or writes that. There is no comparable word do-badder.) But even adding do gooder and do-gooder doesn’t account for all the usage of gooder.
Do-gooder should be a positive word, but is actually semi-negative. Dictionary.com defines it as ‘a well-intentioned but naive and often ineffectual social or political reformer’. (Possibly it should include ‘moral’ as well.) C S Lewis put into the pen of the demon Screwtape: ‘She’s the sort of woman who lives for others – you can tell the others by their hunted expression.’ (I’m suprised to find that this is the first time I’ve quoted Lewis in this blog.)
There’s also the issue of farther and farthest v further and furthest, but in class I’m prepared not to mention that unless and until it actually crops up.