The Common Denominator Fallacy

In political dialog and in news, it is common to assume that people and parties that share the same opinion, or adopt the same position, on some topic, are closely associated. It is also common to assume that sharing a certain goal leads to sharing the same agenda. For example, the foundation of the current “ISIS is Saudi Arabia” propaganda is that ISIS and Saudi Arabia exhibit the same characteristics, therefore they are the same.  The same logic is used to propagate fallacies like “Nationalist parties have a Nazi agenda” and “those who speak against a certain political party really oppose that party”.

The underlying logic for the fallacy in such dialogs is:

  1. A and B share the same n characteristics. 
  2. A plans (or has planned) to do X.
  3. Therefore, B plans to do X as well.

In general, we can compare this reasoning to the idea of denominators in arithmetic, if two numbers share a few denominators, they might not share a certain multiplier. For example, 12 and 24 share all the denominators except 24, but 36 is a multiplier of 12, not of 24. The name “common denominator fallacy” stems from this observation, and the fallacy can be described in an easy to remember phrase like “sharing common denominators does not mean sharing a certain multiplier”.

In Real life, no matter how similar people appear, their own goals or agendas might be completely different, and might change over time. Even if two persons empirically exhibited the exact same behavior in all past experiments, there is no guarantee that they will exhibit the same behavior in the next experiment.

The common denominator fallacy is a type of the association fallacy, similar to guilt by association and honor by association. The major characteristic of the common denominator fallacy is that it implies a faulty future outcome, while the guilt (or honor) by association implies a faulty past outcome in most uses.

For example, associating extreme right parties with the Nazis implies, in addition to the obvious guilt by association outcome, that those right wingers have a Nazi-like agenda on political and social topics. Another example is the implied notion that extreme left parties plan to establish a communist regime. Both examples imply that those parties plan to introduce somebody like Stalin or Hitler in some point in the future. These implied statements are obviously unfounded, and can be either proved or falsified by resorting to the official sources of the mentioned parties.

Similarly, while nationalist parties share similar ideologies, their agendas might be completely different. While a nationalist party in some country might be pro-USA, pro-Europe, and Anti-Russia, another nationalist party might be Anti-USA, Anti-Europe, and Anti-Russia as well.  Therefore, associating national parties might lead to misunderstandings and misconceptions about their political agendas.

The “ISIS = Saudi Arabia” propaganda is one of the most used examples of this fallacy in the current political propaganda scene in the Arabic language. While ISIS and Saudi Arabia share so many characteristics, there is a clear difference in their agendas, Saudi Arabia jails and beheads ISIS supporters (among others), and ISIS publicly vows to destroy Saudi Arabia and all other states in the world (except Israel) in its quest to establish the Islamic Caliphate.  In a future post I will highlight the essential ideological differences between ISIS and Saudi Arabia.

Before making any association between two or more people or parties, differences in their agendas and underlying motives must be carefully studied in order to avoid committing the common denominator fallacy.

Al-Ayham Saleh

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