Appeal to Tradition

This is also known as Argumentum ad antiquitatem.

This is the fallacy in which one argues that a secondary claim is true simply because our ancestors have always done some practice or had certain ideas, or some historical people whom we admire did.

It is possible that Q will have little or nothing to do with P, or the arguer's interest in P may be fleeting or insincere.

Examples

Our society has always embraced smoking, therefore smoking is beneficial to one's health and should be done often.

Our religious group has always stressed that we must only breed with other members of our group (despite the obvious genetic diseases that such a practice leads to), therefore we will continue upholding the practice.

Coffee consumption is traditional in our culture, therefore tea is awful. Tea consumption is traditional in our culture, therefore coffee is awful.

It is traditional to use a ton of sugar and petroleum-based food colorings in foods for children, therefore these are good and all evidence that these cause cancer, ADHD or obesity can be ignored.

Weaknesses

Whether a practice or idea is traditional or non-traditional, that has no logical bearing on whether the secondary claim Q is valid.

People and groups can easily get stuck in a rut and that rut becomes tradition. Therefore you should call into question the originating or instigating circumstances of a given tradition. Just because an established practice is established, that doesn't mean it's good.

You might question whether the proponent is motivated by nostalgia, in which case you could point out that people of every age have felt nostalgia.

Appeal to Tradition may be backed up with Appeal to Authority. Claim Q is true because it's tradition and our elders affirm the truth of claim Q. Fallacy begets fallacy.

If Q has nothing to do with P, you need only point that out. For instance, our culture traditionally serves tea at tea-time, therefore aliens exist. Non-sequitur.

Valid uses

If the tradition is the result of a long process of refinement, careful reasoning, and evidence gathering as for instance the US Constitution is, and claim Q supports the tradition, Appeal to Tradition can be valid. Here the maxim holds that tradition is innovation that sticks.

If the tradition is the result of a long process of self-questioning, evidence gathering, and is malleable as the scientific theory of Evolution is, and claim Q supports it, again Appeal to Tradition can be valid.