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My understanding is that 思われる, in addition to being the passive form of 思う,
can also be used in the sense of "to spontaneously think; to appear".

> 北米では通常スカートは女性がはくものと思われている。  
>  In North America a skirt is thought of as something a woman wears.

In which sense is 思われる being used here? Both seem reasonable:  
Spontaneous: In North America, people spontaneously think a skirt is something
a woman wears. Passive: A skirt is thought of by North Americans as something
a woman wears.

I lean more towards the first one, since a passive sentence would normally
mark the agent with に i.e.

> スカートは北米人には女性がはくものと思われている。
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2013-09-17T04:10:29.720
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question
grammar
Interpretation of 思われる (spontaneous or passive?)
From your last comment (emphasis added):

> a verb that indicates what the **speaker/writer** feels spontaneously

This does not match the translation you give for "spontaneous:"

> In North America, **people** spontaneously think a skirt is something a
> woman wears.

Do you see the difference? If `思われる` were supposed to indicate the _author's_
thoughts or feelings, a better translation would be

> It seems that in North America skirts are usually worn by women.

However, with no other context, there is nothing to indicate that this is a
personal observation of the author, so the translation

> In North America a skirt is thought of as something a woman wears.

seems most appropriate to me.

Also note that it is quite common to leave out the agent in passive sentences.
Just a few examples:

> 今日学校でいじめられた。  
>  I got bullied at school today.
>
> 政府は倒された。  
>  The government was overthrown.
>
> 彼は殺された。  
>  He was killed.
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2013-09-17T19:59:35.477
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answer
The 自発 (spontaneous) 思われる is a static verb (like いる and ある) and cannot take
the ている form. So the 思われている in the example is definitely a passive.

The passive 思われる can take the 思われている form based on tense/aspect.
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answer