The good folks at Campaign Monitor have crunched some numbers on the sorts of words which, when present in an email's subject line, are most likely to get that email opened.
Analyzing the behaviour of nearly 4 billion (!) recipients across 360,872 campaigns, the Campaign Monitor team reached the following conclusions:
Personalization rules - It's said that a person's favorite word is their own name and now, we have the facts to back this up. Without a doubt, subject lines that are personally addressed, do the best - just don't forget to test!
Personal pronouns work, too - Don't have your subscribers' names handy? The popularity of "We" and "You/Your" shows that subject lines that make some kind of appeal to the reader are more likely to get a response.
Make it timely - Another trend to note is that subject lines that feature dates, or urgency seem to perform better than those that don't. Holding your subscribers to a date to act, or letting them know that you're waiting on them (with say, "Invitation") can be a very persuasive tactic.
Be exciting! Finally, we noticed while doing this research is that subject lines that end with an exclamation mark tend to result in more opens than those that don't. While we don't encourage everyone to go overboard with enthusiastic exclamations (!!), it's certainly interesting to see how a little extra energy in your subject impacts email behavior.
Big thanks to Campaign Monitor for their analysis!