Decision rationale: The why and the wherefore

By Gordon Rugg

People are usually able to give you reasons for the things that they do. Sometimes those reasons make perfect sense; sometimes they make sense once you understand the background; other times, you’re left wondering what one earth is going on in the person’s head. There’s also the issue of whether those reasons bear any relation to reality, but that’s another story.

This article is about how one apparently pointless superstition can be traced back to a perfectly sensible piece of evidence-based reasoning that subsequently spiralled off into a very different direction. It involves the ancient Roman practice of examining the livers of sacrificed sheep and poultry as a way of predicting the future.

So what does this practice have to tell us about how people make decisions today? Actually, quite a lot. It’s a good illustration of some fundamental points that are as important now as they were over two thousand years ago, when this bronze model of a liver was created to help Roman fortune tellers assess the omens before a major decision.


At first sight, trying to predict the future by examining the entrails of sacrificed animals doesn’t exactly look like the most rational of ideas. However, when you dig into classical Roman literature, you discover something that casts a whole new light on the subject. It’s in Vitruvius, The Ten Books On Architecture, which was written during the reign of Augustus, about 15 BC. The text below is from the Morgan translation, on Project Gutenberg. Here’s what Vitruvius has to say about animal livers, in Chapter IV: The site of a city.

9. I cannot too strongly insist upon the need of a return to the method of old times. Our ancestors, when about to build a town or an army post, sacrificed some of the cattle that were wont to feed on the site proposed and examined their livers. If the livers of the first victims were dark-coloured or abnormal, they sacrificed others, to see whether the fault was due to disease or their food. They never began to build defensive works in a place until after they had made many such trials and satisfied themselves that good water and food had made the liver sound and firm. If they continued to find it abnormal, they argued from this that the food and water supply found in such a place would be just as unhealthy for man, and so they moved away and changed to another neighbourhood, healthfulness being their chief object.

That’s about as hard-headed and practical as it gets. So how did this spin off into fortune telling? There are several issues involved. I’ve picked out a few that are still as relevant today as they were back in ancient Rome.

Wonky causal models

Sometimes people end up with weird conclusions because the assumptions or the reasoning that they’re working with have gone wrong somewhere along the way. In the case of the Romans, the initial reasoning (it’s a bad idea to build a town in an unhealthy environment) is fine. However, if you argue that the particular environment is unhealthy because the gods are angry, that’s a more dubious assumption. It’s even more dubious to argue that diseased animal livers are a sign of divine anger in general, not just of an unhealthy environment.

Human beings in general are bad at logical reasoning; that’s a central theme of my book Blind Spot, and the reasons for this weakness are complex enough to fill numerous books. In consequence of this, there are plenty of cases of people reaching conclusions that now look odd, but that appeared at the time to make perfect sense. The essays of the late Stephen Jay Gould contain some beautiful examples of this.

Hiding in the herd: Twenty million lemmings can’t all be wrong

At a more cynical level, it often makes sense to play along with widespread behaviours even if you think they’re pointless and irrational, simply because that’s a lower-risk strategy than drawing attention to yourself by asking critical questions. There’s a substantial empirical literature on this, much of it using concepts from game theory, decision theory and/or evolutionary ecology to test hypotheses about the viability of different strategies.

Covering your back

Hiding in the herd is a passive strategy that reduces the risk of encountering trouble, but doesn’t offer much help if trouble does arise. Pointing out that everyone else was breaking the speed limit on a particular stretch of road, for instance, isn’t the strongest of cases if you’ve been pulled over for speeding.

A more effective strategy for handling trouble is to have a scapegoat ready to hand. In the modern business world, that’s a common reason for hiring consultants. If things go well, the manager takes the credit; if things go badly, they fire the consultant.

This is one reason that a lot of businesses used to hire consultants to advise on recruitment, using methods that had been repeatedly shown to have little or no predictive value, such as graphology. At a practical level, this didn’t look like a sensible business decision. At the level of organisational politics, however, this was a very sensible decision, that insulated the manager doing the hiring from the risk of being blamed for a bad appointment.

Forgetting the original reason

Finally, some behaviours start off with a reason that everybody knows at the time, but that gets forgotten as the years go by. That’s a fine, rich topic, although many of the widely-quoted examples are actually urban legends…


That’s a quick skim through some of the issues behind decision rationale. It’s a topic to which we’ll return repeatedly in future articles. A closely related issue is the difference between official and unofficial versions of reality, the topic of some fascinating work by researchers such as Goffman and Argyris, which will also be the topic of future articles.

Notes and links

An overview of Vitruvius’ Ten Books On Architecture:

The Project Gutenberg page for Vitruvius’ Ten Books On Architecture:

Blind Spot is available on Amazon here:


4 thoughts on “Decision rationale: The why and the wherefore

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