Interesting 2010 study, with conclusions that poker players will fold more often than they should to smiling, "trustworthy" faces. In the conclusions, it makes the point that the real cause could actually be that the increased competitive, calling actions are inspired by the un-friendly and neutral-looking faces.

In my experience, players who are perceived as more friendly at the table do get more folds. But this is also often because the more friendly people are not playing a really aggressive game and are more likely to be betting for value than most.

It made me think for a few minutes about whether it could have implications for picking an optimal demeanor at the table, but seeing as how I want action a good amount of the time, it wouldn't make sense to adopt a more "smiley" persona, and I definitely wouldn't want to switch between smiling and not smiling based on whether I wanted a call, as good players would pick up on that.

Interestingly enough, there is a common tell that many players exhibit where they wear a tiny smile when they are bluffing; that might be an instinctual attempt to use this kind of deeply-ingrained information in their favor, but they are not using it in a balanced way.

Here's the link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0011663

In a cash game you want to be friendly. Otherwise your action can dry up. I think you have to build your game around getting those extra folds. Mo friendly mo folds mo money mo bitches mo problems ^^

it depends on the situation again in poker there's no rules if you know what i'm saying

A few things:

1. In their model, villain's face is being used as a proxy for playing style, which is an unobserved variable. There is no feedback information after a hand, so the subjects do not gain information about the opponents after repeated trials. This just tells me that people use Bayesian weighting and are probably correctly assigning a tighter range to a more trustworthy face. It seems like a rather obvious result. There's no way to determine what happens if the trustworthy face is known to play an aggressive style.

2. 12 of the 14 subjects were poker novices, so I'm not sure how generalizable the findings are.

Anecdotally, it seems like I get way more action when I dress like an "internet punk" and maintain a very neutral facial expression vs. dressing professional and being nice and chatty. Not a scientific evaluation though.