Theory: Casual emails have been canibalized by Facebook and messaging products, leaving just the presumably more long winded emails that are more serious in tone.
If the lengthening indicates that vague subject lines such as "Hello!" or "Changes" is being replaced by something more specific like "Meeting at 3 p.m. agenda changed", then it's a good thing. I also like all the info in the subject line, such as "Free pizza in the break room <eom>"
Looking at my inbox I would suspect the major culprit to be marketing mail. By far the longest subject lines come from marketing messages, most of which are using the subject line to make their plea or call to action. Emails from real people appear to have short subject lines. From my own experience, too, I tend to avoid wordy subject lines, saving detail for the message body.<p>The listserve option seems about the least likely to me simply because listserves have existed for a very long time. Unless mailing list names became much longer I would suspect that any changes in subject length would simply be in following larger trends (for example, the larger screens option).
The general trend upwards is interesting, but is anyone else more intrigued by the dip in 2008 followed by the spike in 2010?<p>My hypothesis is that this is being heavily influenced by automated emails from certain websites that the author is using over time.<p>And this could explain the general trend; that proportionally hand-written emails make up a smaller portion of our messages these days -- a quick glance at my inbox and automated 'notifications' are generally more likely to have longer subject lines.
It would be interesting to look at the data for how many 1, 2, and 3 word subject lines there are over time. Maybe fewer "Hello" or "Thank you" subject lines as more people got used to the idea that it's good to be specific and informative in the email subject.
Can confirm firsthand. Started a new job less than a year ago. My new boss shuns/avoids/fears IM, so he'll send me emails with the message contained in the subject line. Strikes me as a little backwards, but he's 50 years old and prefers it to IM...