So to design a better controlled experiment, the OP should write a statistically significant number of emails (>100...maybe 1000+) without sending them. An app should perform a double-blind study by randomly choosing half those emails to go to multiple recipients (all but one being fake) and choosing the other half to go to one recipient only.

Anyone see any holes in this approach?

"without sending them" is unnecessary. Modify your email client to randomly choose (AFTER you write your email) whether or not to CC Alex. Or just flip a coin after writing it and before hitting send.

How about the idea of writing 100+ e-mails without sending them?

You'd be better off having a mail plug-in randomly selected whether to add a cc at the time of sending rather than hoarding them.

Your mechanism is valid as a trial but not particularly workable in the real world.

You might want to also control for factors such whether the mail is internal or external (assuming it's a company), message length (longer messages might be more or less likely to be properly read and responded to), message importance (you'd expect more replies to higher importance messages), attachments and so on.

Ack. Always address one of the recipients by name in the body of the email to ensure that it's not SEP'ed.

isn't there a name for that? something like diffusion of responsibility. the same reason we don't help one person in trouble when there's more than one potential helper...

The bystander effect/Genovese syndrome: http://en.wikipedia.org/wiki/Bystander_effect

Perhaps the effect is quite different when there is just one other bystander, as opposed to a whole crowd of them.

Yes, but there's a critical difference. In this case there's a clear recipient - someone asked to do something/reply. In the case of the Genovese effect, no-one is specifically asked to do anything, and no-one does.

Correct: http://en.wikipedia.org/wiki/Diffusion_of_responsibility

You mean like this: http://www2.selu.edu/Academics/Faculty/scraig/gansberg.html

But this technically isn't diffusion. It's concentration of responsibility.

Well, I've always tried to construct my e-mails to the best of my ability. I didn't noticed any change in my content - it has all been pretty standard.

Unfortunately you not noticing any change in the content doesn't make for very reliable or convincing statistical results. Thus the suggestions for making the experiment blind to you by choosing whether or not to CC after you compose the email.