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Decision-Making
Our Decision-Making Doesn’t Always Fit the Rules of Logic
We often rely on flawed reasoning when making decisions.
Posted November 22, 2021 Reviewed by Ekua Hagan
Key points
- In deductive reasoning, if the premises are true, one can logically conclude that the claim itself must be true.
- An inductive argument takes one or more statements and uses them to specify a probable conclusion.
- Not paying attention to the strength of one's reasoning or the quality of the evidence behind it leads to several decision-making errors.
The defendant didn’t tell his side of the story after he was arrested.
Innocent people never invoke their Fifth Amendment right to remain silent.
Therefore, the defendant is guilty[1].
This was essentially an argument made during the Kyle Rittenhouse trial. An argument is nothing but a series of statements (premises) that allow us to infer a conclusion (or claim)[2]. Arguments like the one above are seen regularly on social media sites whenever a trial of any regional or national importance occurs. The problem is that the argument is flawed (and any lawyer can tell you that). To better understand why, we need to define the two basic types of arguments: deductive and inductive[3].
Deductive Reasoning
The logical expression presented at the beginning was a form of deductive reasoning. In deductive reasoning, the inference made from the premises to the conclusion is airtight. That is, if the premises are true, we can logically conclude that the claim itself must be true. Such arguments are considered logically valid. Consider the following:
All healthy trees have leaves in the summer.
An oak is a type of tree.
Therefore, a healthy oak tree has leaves in the summer.
If all healthy trees have leaves in the summer and if an oak is a type of tree, then a healthy oak must have leaves in the summer. Hence, the argument itself is valid, regardless of the actual truth of the premises.
Now, consider an alternative argument:
An oak is a type of tree.
A healthy oak has leaves in the summer.
Therefore, all healthy trees have leaves in the summer.
If an oak is a type of tree and if a healthy oak has leaves, it does not logically lead to the conclusion that all healthy trees have leaves in the summer. Therefore, this argument lacks validity. We cannot use the premises to argue, for example, that a healthy elm tree is going to have leaves in the summer.
The validity of the argument, though, is merely the determination of whether the conclusion logically follows the premises if the premises are true. Premises, though, aren’t always true; there may not even be any evidence to support their truth. In such cases, no matter how valid the deductive argument is, the argument would be said to be unsound.
Let’s return to the argument presented at the beginning:
The defendant didn’t tell his side of the story after he was arrested.
Innocent people never invoke their Fifth Amendment right to remain silent.
Therefore, the defendant is guilty.
If we assume that the defendant didn’t tell his side of the story after he was arrested and if we assume that innocent people never invoke their Fifth Amendment rights to remain silent, then we must logically conclude that the defendant is guilty. Therefore, the argument itself is valid.
However, there’s a significant problem with the second premise. It cannot be stated with any truth that “innocent people never invoke their 5th Amendment right to remain silent.” In fact, innocent people do exercise their right to remain silent, and lawyers will often advise their clients to exercise their fifth amendment rights regardless of their guilt or innocence. Since the second premise is untrue, the deductive argument is unsound[4].
We can, however, try to restructure our argument so it’s inductive rather than deductive.
Inductive Reasoning
Let’s alter the argument somewhat and state the following:
The defendant didn’t tell his side of the story after he was arrested.
Innocent people seldom invoke their 5th Amendment rights to remain silent.
Therefore, the defendant is probably guilty[5].
This type of argument would no longer rely on deductive reasoning (because the conclusion isn’t guaranteed to follow) but would instead be considered inductive reasoning[6]. Inductive reasoning is not focused on an airtight path from the premises to the conclusion. Instead, an inductive argument takes one or more statements and uses them to specify a probable conclusion. Rather than evaluating an inductive argument based on validity and soundness, an inductive argument is evaluated based on its strength (if the premises are assumed true, then how probable is the conclusion?) and cogence (a strong argument that possesses true premises).
A reading of the inductive argument would suggest it’s a strong argument following much the same logic we employed for the validity of a deductive argument. However, the argument itself isn’t a cogent argument because the second premise lacks evidence to support it[7].
There are a variety of ways to formulate inductive arguments, which generally vary based on the types of evidence presented in the premises (e.g., anecdotes, statistics) and the type of claim being argued (e.g., generalization vs. prediction vs. causation), and inductive argument strength will vary based on how probable the conclusion is to follow the premises provided[8].
Where We Make Errors
Although we all reason, we often fail to pay attention to the strength of our reasoning or the quality of the evidence underlying it. There are several ways this tends to impact our decision-making, but I want to emphasize four in particular.
- We tend to believe we’re arguing deductively when we’re really arguing inductively. As evidenced in the initial example for this post, many people believe their argument is airtight. This can lead to a faulty level of confidence, which may pose unintended consequences later.
- We fail to identify a “probability” standard. Even if we recognize we are arguing inductively, how we choose to act based on an inductive argument may vary based on how probable the conclusion is. If, for example, you conclude that a particular investment option is probably going to pay off, your action may be different if you conclude such an outcome is 85% probable as opposed to 60% probable, especially if the investment requires what you consider to be a large amount of money. Given that every probable conclusion also has at least one less probable (but possible) conclusion, there is some inherent risk when acting based on our conclusion (sometimes, a highly probable outcome is still not probable enough).
- We commit logical fallacies. Logical fallacies are errors in logic and are often discussed more in the context of deductive arguments than inductive ones (though any type of argument can be fallacious). Fallacious reasoning renders a deductive argument invalid (and thus unsound), but it also weakens inductive arguments, potentially to the point of being considered fatally flawed.
- We fail to consider the credibility of the evidence underlying the premises. It’s quite easy to look for evidence to support the premises we use to make an argument (this is, of course, a hallmark of confirmation bias). However, doing so will result in an argument that may be easily refuted (or fatally flawed), so it can be quite helpful to adjust the confidence we have in our conclusion so it is consistent with the existing evidence.
There are no magic formulas for how to avoid these traps or how big an issue each trap is. All situations are different, and the consequences of relying on flawed reasoning will vary (situations with greater consequences may benefit more from cogent or sound arguments than situations with fewer consequences). Learning to better understand our own thought processes and how we implicitly and explicitly formulate arguments (what is known as metacognition) may be the best way to improve our decision-making.
References
[1] This is not a direct quote, but it was an insinuation made by the prosecution and one echoed repeatedly on various social media outlets. We can rework the premises in any number of ways to still derive a valid deductive argument. For example, we could change the second premise to “Only those who are guilty invoke their 5th Amendment rights.”
[2] We can contrast this with many disagreements we refer to as arguments, which often possess any number of flaws that make them illogical.
[3] A third type, abductive, is also sometimes discussed. It appears to be more a form of inductive reasoning focused on sensemaking in the absence of strong premises (i.e., a best guess). Given the lack of agreement surrounding this topic, it’s well beyond the scope of what I can write here.
[4] The conclusion itself may be accurate. However, the premises, as stated, do not logically lead to that conclusion.
[5] It’s not uncommon to see the conclusion of inductive arguments listed without the term probably. I would contend, though, that the downside of such conventions is they often communicate a degree of overconfidence in the conclusion that may or may not be intended.
[6] Wikipedia defines inductive arguments as reasoning from the specific to the general. While such reasoning is inductive, many inductive arguments do not have that form (as discussed by the Internet Encyclopedia of Philosophy), and even some of the examples in the Wikipedia entry are inconsistent with this definition.
[7] Lawyers can tell you that the Supreme Court has ruled that invoking one’s 5th Amendment rights cannot be used as evidence of one’s guilt or innocence in a criminal trial.
[8] The definition of probable can be ill-defined. Technically, a standard of more likely than not (i.e., 51% likely) is a probable conclusion. Depending on the situation, such an argument may be considered strong enough or it may be considered weak. I’ve alluded to this in past posts, including in the context of trade-offs, science vs. lived experience, and our predictive algorithms.
