I was recently talking with an acquaintance who makes custom wedding dresses. The lead time for making dresses is typically several months and tailoring a dress that's going to fit someone 3-4 months after the initial measurements are made can be challenging. Most brides-to-be desire to lose some weight before the big day and typically share a target weight/size with her..."Make it a size smaller because I'll be 20 pounds lighter on the day of the wedding".
This woman's been doing it for so long that she's learned to ignore what these brides say will happen and to plan for what actually ends up happening. The outcome is pretty simple, she says; as the wedding day approaches, thin women get thinner and the heavier women get heavier. The hypothesis here (expressed by the dress maker) is that the weight loss/gain depends on how these women deal with the stress of the event: thin women don't eat or lose their appetites when stressed while heavier women eat in response to stress.
Aside from how general a statement you can make about relation of the stress/eating/weight factors, the fact that she's able to accurately size dresses based on this simple rule is another reminder of how misleading it can be to rely on asking people about their potential behavior. As a web designer, one the most valuable things I learned when building sites was that watching people use prototypes or web sites was way more useful than asking them what features they wanted.
