Why people share passwords
People often collaborate in ways we don't necessarily expect, such as sharing passwords. I still think most people prefer to do the right thing, but not enough to go out of their way to make it happen. Convenience rules, and people use systems in ways that their designers never intended. As Jon Udell points out, we need to understand why people do what they do. Is it for something nefarious like an alibi? Or does it perhaps serve a legitimate purpose?
Once again, the fundamental issue is trust. Computer trust models do a very poor job of mapping people's trust models. Let's look at a typically complicated trust model in the physical world. You are going on a trip. You give a neighbor a key to bring in your mail. You trust this neighbor with the run of your house more than you trust the world not to break in to your house or steal your mail as they see it piling up in your box.
Now add the fact that your daughter is at camp and can write you snail mail, but can not email you. Postal mail will not reach you in time on your journey, so your daughter sends the mail to the house. If you want to know that your daughter is okay and having a good time, you may authorize that neighbor to open letters from your daughter and call or email you the contents of her letters, since you have no other way of knowing how she's doing. This is a different level of trust than simply bringing in the mail.
How does this sharing of your physical mail "password" differ from sharing of your email password? Conceptually, I don't think it does. How do we facilitate the end of sharing information without the potentially dangerous means? We need to develop and implement complicated and nuanced trust models that can be used easily and simply. A big task, but a necessary one.