The other night (after hearing someone on TV say "smoothen"), I noticed that a fair number of Anglo-Saxon-derived adjectives tend to come in pairs where the more "distinguished" or "active" adjective becomes a verb by adding -en while its opposite adjective is the same as the verb form.
|fasten, tighten||loose (but also "loosen")|
My question is: Is there a known or theorized linguistic reason for this pattern? I have at least three hypotheses: (1) The -en marker connotes human activity: one must act positively to brighten and sharpen things, but they will dim and dull on their own. (2) The marked words are more frequent and/or more important; it's useful to distinguish [please] brighten from [it is] bright, but the distinction between [please] dull and [it is] dull is less practically relevant and therefore verbally unmarked. (3) This is entirely a case of confirmation bias.
Of course the pattern isn't universal; I can think of more exceptions to the rule than I can followers. I'll list them here, just in case it's useful to someone and/or someone thinks of a reason to move one up into the table above.
|heat (*hotten)||warm, cool|
|speed (but cf. modern "quicken")||slow|
|straighten||crook, bend, curve, curl|
|moisten, dampen||wet (*wetten), dry (*dryen)|
|heighten (*tallen), lengthen (*longen)||shorten|
|— (*heavyen, *weighten)||lighten|