Our Gemara on Amud Aleph mentions various illnesses that befall a person who does not keep parts of his body or his clothes clean. Some of it seems to be psychological in origin, that is that it induces a sense of disorder, and possibly even mental illness.

It is common sense to assume that one’s mode of dress influences their attitude, for better or for worse. What does the research say about this?

We have the general psychological and cognitive principle known as priming. Priming is when there is a particular mindset induced either by visual, auditory, or spoken stimuli, that predisposes one along a particular mode of thought, or feeling. For example, in Daniel Kahneman’s “Thinking, Fast and Slow” he explains:

If you have recently seen or heard the word EAT, you are temporarily more likely to complete the word fragment SO_P as SOUP than as SOAP. 

Priming is not restricted to concepts and words. You cannot know this from conscious experience, of course, but you must accept the alien idea that your actions and your emotions can be primed by events of which you are not even aware. In an experiment that became an instant classic, the psychologist John Bargh and his collaborators asked students at New York University -- most aged eighteen to twenty-two -- to assemble four-word sentences from a set of five words (for example, 'finds he it yellow instantly'). For one group of students, half the scrambled sentences contained words associated with the elderly, such as Florida, forgetful, bald, gray, or wrinkle. When they had completed that task, the young participants were sent out to do another experiment in an office down the hall. That short walk was what the experiment was about. The researchers unobtrusively measured the time it took people to get from one end of the corridor to the other. As Bargh had predicted, the young people who had fashioned a sentence from words with an elderly theme walked down the hallway significantly more slowly than the others.

Reciprocal links are common in the associative network. For example, being amused tends to make you smile, and smiling tends to make you feel amused. Go ahead and take a pencil, and hold it between your teeth for a few seconds with the eraser pointing to your right and the point to your left. You were probably unaware that this action forced your face into a smile. College students were asked to rate the humor of cartoons from Gary Larson’s The Far Side while holding a pencil in their mouth. Those who were 'smiling' (without any awareness of doing so) found the cartoons funnier than did those who were 'frowning.' 

But aside from this general concept of priming, which seems to have been proven, there is another experiment, specifically around mode of dress conducted by researchers Adam and Galinsky (Journal of Experimental Social Psychology Volume 48, Issue 4, July 2012, Pages 918-925) discover the following effect:

By asking participants to wear a lab coat described as a doctor's coat, it increased sustained attention compared to wearing a lab coat described as a painter's coat. Furthermore, compared to simply seeing or even identifying with a lab coat described as a doctor's coat. Thus, their research suggests a basic principle of, what they call, enclothed cognition—it depends on both the symbolic meaning and the physical experience of wearing the clothes.

We are in a society where hierarchy and authority is watered down, and old standards of dignity and dress are replaced by valuing comfort and casualness. I do think such sensitivities have their place because we can’t push children (or adults) to do something that is to the extreme of their cultural context, as it will just make them resentful. Nevertheless, we should be mindful that certain standards of dress do influence a sense of dignity, self, and pride.