Most people will say that appearance does not have anything to do with a person's personality and I'll be one of the first to differ. Although it's not always a sure-shot way of describing someone upon looks, it can be pretty accurate at times.
First we can take into consideration the styles which people dress like. We can usually classify them as Rapper/Urban, Surfer/Skater, Emo/Punk, Metrosexual, Sporty and Geek/Nerd. Same goes for both men and women except that you would change Metrosexual for what we in Puerto Rico have a slang for is "come mierda" and I do not know of a direct translation for English but it's the type of girls who just want the most expensive straight out of design fashion outfits.
** Note: The literal translation of "come mierda" is shit eater, which could make sense regarding the shitty attitude that these kind of people usually have towards others, usually thinking of others as inferiors.
What does dress style have to do with personalities? At first I thought not much, but a few friends of mine who are studying Psychology have told me that it does. A person thinks, acts and therefore expresses themselves a certain way. This certain expression can usually be seen in the person's way of clothing. Now don't misunderstand this, the person can show his/her type of personality via the clothing being worn, not the clothing worn defining the type of personality.
EX: A rapper type of guy would dress like a rapper. If he one day decides to dress emo, he'd still be a rapper.
Now what does all these clothing types have to do with personality? It gives us a general outline of your way of thought and who you get along with best. Rappers usually think one way, Emos another and so on... it also tells which which kind of people you hang around it, what you best describe yourself with the best and therefore what you like the best.
So clothing can give us a general outline of the person, what about more specific stuff like whether the person is very social or not, a party animal or a small get together type of guy, really outgoing or more reserved, funny or unfunny... etc.
This is a bit harder to understand and explain. I'll just give a few examples here since I haven't been explained this before and I'll just go on pure experience, so this may or may not be correct.
1) Laid-back people usually will dress a little bit more informal than what is considered acceptable*. They'd go a few extra days without shaving, don't care if what they wear matches or if it looks goofy. Hell, I've seen times when they'd only get a haircut and shave when they got a big thing for which they need to look good or else they may loose job or something related. Laid-back people aren't fond of details, hence why they are a bit less informal. Usually not very organized.
Ex: This is the guy you see going to College in surfing shorts and a random T-shirt, can be sleeveless, with unprepared hair and facial hair which should have been shaved a few days back.
*I assume most of you are still in high school and don't know much about dress etiquette, haven't experienced it or think you don't have to apply by it's "rules". Once in College one really gets a feel for this. You don't give presentations in your bathing outfit because right after it you're going to get drunk at the beach, you don't go in a button shirt and dockers to class, the weather can dictate what you wear and when you will go out with someone you like you will usually dress to impress. Each occasion has it's right and wrong dress.
2) People who go right down to details, well, are always looking after their details. Men get a haircut every week or so and you'll never see that 5 o'clock shadow. For women they'd always be on their best, often with perfect looking makeup. Usually dressing a little bit more formal than required. These people tend to be quite organized.
There are a lot more of these around, I just can't go naming them all because I'm no expert here. The theory is that a person usually expresses themselves through their clothing and this shows their personality. The general argument against it is that you can't judge a book by it's cover. Well, would you put a cover with a nice looking playground with children happily playing on a horror novel? No. Generally when you see something way out of line, it's that the person could be insecure. Also note that the human mind is spontaneous and can change at will, so pretty much every theory about it isn't always entirely correct or foolproof.
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