I would accept that many of them behave like humans born on earth would, but that must be an artefact of poor translation or our poor understanding of their motives and objectives [Watsonian] or lack of vision on the writers part [Doylist - to borrow some terms from the Bujold mailing list - think 'as written by' Dr Watson or by Arthur Conan Doyle].
Source: wiktionary
Roddenberry's official explaination^([sic]) was "They always looked like that. We just didn't have the budjet^([sic]) in the 1960's to show them properly."ΒΆ This is one case where I prefer the Doylist explaination^([sic]).
Source: wiktionary
While fans recognize and do engage in Doylist readings, they tend to find Watsonian readings more engaging. Fan fiction writers in particular engage with the text intradiegetically.
Source: wiktionary
T. Raymond
One can imagine the devout Doylist wringing his hands over every fresh appearance of Sir Arthur in the character of an exponent of spiritualism.
Source: wiktionary