Also called It-Ought Fallacy.
The Appeal to Common Practice is a fallacy with the following structure:
The basic idea behind the fallacy is that the fact that most people do a is used as "evidence" to support the action or practice. It is a fallacy because the mere fact that most people do something does not make it correct, moral, justified, or reasonable:
This argument rests heavily on the principle of relevant difference. On this principle two people, p1 and p2, can only be treated differently if and only if there is a relevant difference between them. For example, it would be fine for me to give a better grade to p1 than p2 if p1 did better work than p2. However, it would be wrong of me to give p1 a better grade than p2 simply because p1 has red hair and p2 has blonde hair. An appeal to fair play, which might seem to be an appeal to common practice, need not be a fallacy.
There might be some cases in which the fact that most people accept a as moral entails that a is moral (cultural relativism). For example, one view of morality is that morality is relative to the practices of a culture, time, person, etc. If what is moral is determined by what is commonly practiced, then this argument:
Director Jones is in charge of running a state waste management program. When it is found that the program is rife with corruption, Jones says "This program has its problems, but nothing goes on in this program that doesn't go on in all state programs."
"Yeah, I know some people say that cheating on tests is wrong. But we all know that everyone does it, so it's okay."
"Sure, some people buy into that equality crap. However, we know that everyone pays women less then men. It's okay, too. Since everyone does it, it can't really be wrong."
"There is nothing wrong with requiring multicultural classes, even at the expense of core subjects. After all, all of the universities and colleges are pushing multiculturalism."
A woman working in an office might say "the men who do the same job as me get paid more than I do, so it would be right for me to get paid the same as them." This would not be a fallacy as long as there was no relevant difference between her and the men (in terms of ability, experience, hours worked, etc.).