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I've caught myself writing (typing) "possible" instead of "possibly" a few times over the past few days, while I do intend to write "possibly". Only upon rereading the sentence I notice my mistake.

It is not a typo. I am able to touch-type on a qwerty layout on which 'e' and 'y' are both written using a different hand, and different fingers. The keys are two keys apart from each other.

A simple google search for "psychology writing wrong words" didn't show up any immediate relevant results. I did read about a Freudian Slip, but it seems highly unlikely I would make such a mistake due to an "unconscious ('dynamically repressed'), subdued, wish, conflict, or train of thought".

Is there any psychological phenomenon explaining why I would make such an error?

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These (non-academic) articles cover some of why your brain fails to pick up those errors and site some good sources:… – Ben Brocka Jan 23 '12 at 20:09
On a personal note: I have had this happen 3 times in the last 48 hours. I think sleep deprivation is the contributing factor but it is scary. Examples: I wrote 'Brad' instead of 'Brian'. I wrote 'thing' instead of 'think' and I wrote 'ass' instead of 'all'. I also confused tax day April 15 with April 13 and struggled to recall my work cellphone. I feel like I am losing it. I have multiple degrees and am fairly put together so this has been really disturbing. – user4773 Apr 10 '14 at 4:00
I have had the same problem lately, mixing up simple words like their, there and they're for example. I know exactly which should be used where so that is not the issue with me either. Also when I read over what I've written my mistake jumps out at me and I can't believe I have made such a basic error. This is only a problem I've encountered in the last few months. I am wondering whether it could be the beginning of Alzheimer's, as it does sit in my family. From what I understand Alzheimer's begins in the basal ganglia which is the same part of the brain used for writing. – user4886 Apr 24 '14 at 13:42
I do this too, and it drives me bonkers. It is even worse for me because when I do it, it's not with a related work. I type a different word starting with the same letter and usually with the same number of characters: "instead" instead of "inside" was the newest example in an email to my wife just today. And then there's times I just miss a word all together. – scott.korin May 15 '14 at 15:47
I do make a lot of errors too when typing especially when I am in a hurry and not paying much attention. But when I write with a pen as opposed to typing, I hardly make such errors. May be my poor typing skills have something to do with it. – Noah Feb 9 at 1:29

The speech error taxonomy on Wikipedia that Jeromy Anglim links to in his answer is pretty comprehensive. If you're interested in learning more, I would suggest reading some articles by Gary Dell (e.g., Dell, 1986). He is, in my opinion, the expert in this domain. He has used neural networks to explain speech errors of different types.

When mentally planning the next word in a sentence, we must choose the appropriate lemma. By selecting the units in a neural network that correspond to a certain lemma, it spreads activation to the corresponding morphemes, which in turn spreads activation to its corresponding phonemes. When the activation of a phoneme unit exceeds a certain threshold, it is selected for utterance (e.g., spreads activation to the motor units that allow us to speak).

However, this model is susceptible to retroactive and proactive interference: if we have just spoken another word that requires a different morpheme, its activation may exceed that of the target morpheme because of (for instance) undue attention or neural noise.

The same model can predict typographic errors as well: phonemes spread activation to orthographic units, which spread activation to the proper motor units which control our fingers.

Interestingly, speech errors rarely violate the rules of syntax, morphology, or phonology (even though they may make little sense semantically). Psycholinguists can use this fact in order to infer the structure of linguistic rules without having direct, conscious access to them.

Dell (1986). A Spreading-Activation Theory of Retrieval in Sentence Production. Psych Review, 93, 283-321. Retrieved from

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I know the model of Dell, but I've never heard of the link with finger movements. Can you provide an article about that? – Mien Feb 4 '12 at 0:17
@Mien I don't know of a paper that specifically looks at that aspect (though there may be one-- I haven't looked). However, 1) if you're willing to accept that Dell's model affects your tongue and jaw muscles, it seems a small leap that it would affect your fingers as well; 2) Botvinick & Plaut (2004) present a popular connectionist model of motor action selection that references Dell several times-- probably a good place to start. – Jeff Feb 14 '12 at 1:40
I was just rereading this answer, given that another user posted the same question. After a second read I notice that I actually do not make the speech error of saying "possible" instead of "possibly". Wouldn't that invalidate this answer? (hence the unaccept) – Steven Jeuris Sep 12 '13 at 21:40
I don't believe this answers the question. For example, why do I accidentally type "it's" when I know it should be "its"? There is nothing in phonemes or morphemes that would cause that kind of mistake. – mawcsco Sep 12 '13 at 21:45

A few thoughts (this is not my area):

  • This article on speech errors on Wikipedia is informative. The article provides a review types of speech errors. The classification of speech errors is presumably similar to writing errors.
  • What I take from the article, and other research on errors, is that there is plenty of structure to errors. I think an information processing perspective would be more helpful in explaining writing errors than a Freudian perspective.
  • Taking an information processing perspective on your specific example ("possible" versus "possibly"), it makes sense that these words could be confused on the basis of their semantic and spelling similarity.
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As an addition to Jeromy's answer, I would like to point out that I sometimes think of them as muscle memory flaws. I have no evidence to back this up, but I'll explain my reasoning. The suffix 'ble' is quite common in English language, so when you type the 'bl' for ('bly' in 'possibly'), without thinking you add the 'e' instead of the 'y', because your fingers are used to the pattern of 'ble'. Of course you can type the (perhaps) less common 'bly' but then you'll need a bit more attention, to inhibit the 'bly'.

Does this sound reasonable?

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Yes it does, and I appreciate your input, but I can't up vote this because this site expects "good, well supported answers". – Steven Jeuris Jan 23 '12 at 18:54
I was just rereading the answer, and actually this might be more plausible. As far as I know I don't make the speech error of saying 'possible' when I meant 'possibly', so even though if "phonemes spread activation to orthographic units" that would not lead to me typing it wrong. – Steven Jeuris Sep 12 '13 at 21:28

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