We often hear things like "Climate change is not real. Jim only supports climate change because his parents are geologists." Or "Christianity is just a made-up belief system. Sue is a just Christian because her grandmother took her to church every Sunday." Since these statements contain a "because" clause, they take the form of an argument. They are not arguments in the sense of two people yelling at each other, as "arguments" are commonly conceived. Rather, an argument is simply the statement of a premise (or premises) and a conclusion. When we make an argument, we are giving reasons for why we think something is true or false. Whether the other person should accept or reject our conclusion depends in large part on our reasoning methodology. If we do not use sound method, that is, if we commit a "fallacy," the conclusion we advance will not follow from the premises. The result is a failed argument, which the other person can simply reject out of hand.
What is a genetic fallacy?
A genetic fallacy is an informal fallacy of logical reasoning. It is a type of "red-herring" or irrelevancy fallacy. We commit a genetic fallacy if we argue against an idea or conclusion based on its origin. The way a person comes to hold a position does not have any bearing on whether the position itself is true or false. To be sure, there are some instances where the origin of belief or testimony can be relevant to the present truth value of a claim, such as in historiography. But in any event, the truth or falsity of a claim must be adjudicated based on the reasons given for supporting (or detracting from) the premises and conclusion.
Returning to one of the previous examples, it is fallacious to argue that Jim's belief that climate change is wrong because of how he came to hold this belief. Climate change is either true or false regardless of whether Jim's parents indoctrinated him with this belief. It could be that Jim formed his position based on independent research or other grounds. Jim's opponent must engage with the argument and support Jim is giving in favor of climate change. Dismissing Jim's claim because of how he was raised, where he lives, his education, and so forth is fallacious. None of these are relevant to the matter at hand. The same thing holds true regarding religious claims or belief in God.
The oft-leveled charge against those who believe in God is that such belief is based on upbringing. These claims are usually more specific, such as that Christians believe in Jesus as Savior because they were brought up to believe this way, just like Muslims were brought up to believe Mohammed is the final prophet of Allah, and so forth. The skeptic will often argue for a summary dismissal of any religious claim on grounds that religious beliefs are just a product of one's upbringing; children growing up in atheist homes will be atheists, and vice versa. Even charitably allowing these claims to be broad generalization does not help them from falling into fallaciousness. The existence of God, the truth of the Bible, or other religious beliefs cannot be falsified based on how adherents came to that belief. The specific truth claims of Christianity must be addressed if one is to falsify it, not how a Christian arrived at his or her conclusion. In the same way, Islam is false because it can be shown that Mohammed was not a prophet, the Quran is not from God, and Islam wrongfully denies the divinity, atoning death, and resurrection of the Lord Jesus Christ. But one cannot claim that Islam is false because the Muslim claiming the Quran is true was raised in an Islamic home or country.
What often gets confused in these discussions is the truth of the belief versus the rationality of the person holding the belief. Religious skeptics often tend to conflate these notions. In cases where the lines of communication are clearly drawn, the person holding a belief might be questioned about the validity of their belief X if they cannot produce any evidence for X or if they came to hold X in a way deemed unreliable. In these cases, the truth of the belief itself is not subject to debate; it is the belief mechanism or rationale of the person that is in question. This is not a genetic fallacy, because it does not aim to falsify the belief. Instead, the interlocutor aims to lower (or eliminate) confidence in the belief or position. Extending this further, it might be that person A is rational believing in God because they offer valid reasons for their belief and person B is irrational because they offer no valid reasons or are shown to have a mental disorder or some other mitigating factor. The debate just described concerns religious epistemology (knowledge) and not religious ontology (existence and truth).
Genetic fallacies are considered a type of "red-herring" fallacy because they result in a diversion from the main issue at hand. Both parties to an argument must avoid this reasoning pitfall. Theists (those who believe in God) should be especially wary of the genetic fallacy, as it can come in many variations. Fruitful discourse can only occur if the truth of the matter is pursued above individual prejudice.
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Truth about Worldview and Apologetics