Why is it that so many people wrongly grasp wineglasses by the bowl rather than the stem? Are they misled by TV and movies, where actors so often do it wrong? And why does everybody seem to know to hold a martini glass by the stem?
Great point about the martini glass. I think that's the clue to your answer. I've scribbled a lot on this topic, mainly, though, to answer questions about how one should hold a wineglass. You already know the answer to that one. You hold it by the stem because it keeps oily fingerprints off the bowl, prevents the transmission of body heat to the liquid and makes it easier to swirl when you want to aerate the wine for nosing. It has also come to be associated with upper-crust decorum, as in keeping one's mouth closed while chewing, but that's a separate issue I'll leave for others to debate.
It's curious that martini drinkers, on the other hand, mostly reach instinctively for the stem. Why? I'd say it's got less to do with fingerprints or warming the liquid than with the architectural imperative of the glass. Try to move a martini around clasping the v-shaped bowl and the booze is likely to end up overboard, spilling onto the table or your lap. It takes balance to keep that liquid steady, and it's much easier, as any cocktail lover knows, to do that by pinching the stem. It's a brilliant design because it compels the drinker to look suave. That's more than can be said for all that fancy wine stemware out there.
The Flavour Principle by Lucy Waverman and Beppi Crosariol (HarperCollins) won top prize for best general English cookbook at the 2014 Taste Canada Food Writing Awards.