For my linguistsics degree, I did a project on why I'm seeing more people saying "on accident" instead of "by accident." I looked at almost a million pieces of writing pulled from news sites, blogs, academic articles and television transcripts. I found almost three hundred cases of "on accident" being used. It was a surprisingly even spread across sources. Even more interesting, I organized the hits by date and tracked an upward swing in use as time goes on. This means that the use of "on accident" is increasing over time, and may eventually supplant and drive out the classic usage of "by accident." I like to call this prepositional shift.
Now, looking at my data and looking at the age ranges of the writers or speakers, the majority of them were under the age of thirty. So I interviewed a panel of people, choosing twenty with a spread of about half above thirty, and half below. Those older than thirty years of age felt "strongly" or "very strongly" that "on accident" was wrong in all cases, and that "by accident" was the only correct phrase. However, those younger than thirty were much less rigorous, with more than half feeling "ambivalent" or "less strongly" about which was correct. This demonstrates a generational link in preposition usage.
When presented with options for the definitions of "by" and "on," we also get some interesting data. For by, there are two main definitions according to the Oxford English Dictionary: 1. Identifying the agent performing an action. Or 2. Indicating the means of achieving something. Whereas "on" has many more definitions, the pertinent ones being 1. To indicate the manner of doing something or 2. To indicate active involvement in a condition or status. By the above definitions, either "by accident" or "on accident" is a correct usage of the term. However, native speakers of English could not successfully define either preposition, instead just choosing one, the other, or both as "sounding correct."
The only evidence for a rule-based shift that I could find was a correlation with the paired phrase for the opposite condition "on purpose." While the younger interviewees were ambivalent about the correctness of "on accident," they uniformly rejected the correctness of the suggested phrase "by purpose." So the shift can only be in one direction according the the native ear, towards the preposition "on."
Whether this means that the particular usage of "by" is becoming archaic or the definition of "on" is expanding is a possible subject of further study using a wider range of phrases. But I found the wider acceptance of "on accident" versus "by accident" to be a fascinating look at how prepositions can shift meaning and usage over time.
So now I'm curious, five years from my initial study (and itching to try the Tumblr poll feature):
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