Decoding human personality

I recently had the opportunity to read a fascinating paper titled “A model for personality at three levels” by Revelle and Condon. I was expecting the article to be rather dry, from which I would need to extract useful facts after a lot of effort. However, it proved to be fantastically “wise”, with deeply penetrating insights into human personality. Written more like a Tolstoy novel than a research paper, it is hands down the most amazing thing I’ve read in recent times.

Levels of individual differences

The authors state that people change with time at an intra-person level, inter-person level and inter-group level. Intra-person refers to the changes within a person, like a sudden surge in anger or hunger, inter-person refers to a relative change between individuals, like a faster decrease in awkwardness in one person than in another which causes that person to speak first in a social situation, and inter-group refers to group level differences, like some majors such as Physics being negatively correlated with agreeableness.

Why are we concerned with trait changes with respect to time, and not stable characteristics? For instance, why are we not concerned with a person’s stable level of anger, as opposed to a “rate of increase of anger”? The authors contend that it is the rate of change of a characteristic, and not the stable value of a characteristic that determines human behaviour. Why is that? Consider the following situation: you’re walking on the road, and you suddenly see an acquaintance from college. There will be a part of you that will want to go talk to them, while the anxious part of you will want to feign ignorance and move on. Whichever tendency rises fastest at that moment will lead to the consequent action. Hence, it is the rate of change of a desire/trait inside a person that determines their actions.

As an aside, consider the following quote:

When others evaluate our reputation, they are evaluating our behavior in critical situations and how it changes across situations. When we think of our identity, we interpret our behavior as the result of our affects and our cognitions.

This is a supremely deep quotation that set me thinking. Yes, people do seem to evaluate the true worth of others only in “critical situations”. If you didn’t help your friend in their time of need, you will forever be thought of as unreliable and “not really a friend”. However, we interpret our behavior as a result of our thoughts and desires (that are not accessible to others), and hence tend to give ourselves far more leeway than others do. Although this quote is not directly related to the rest of the paper, I thought it was a great quote and deserved to be discussed on its own merit.

The authors’ main contention in this paper is that the behaviour of individuals depends majorly on intra-person dynamics, and which tendencies of theirs are reinforced by social approval and encouragement, etc. Of course, this reinforcement only works when an individual is receptive to external opinion and feedback from past performances. For instance, one of the major reasons why I switched to Mathematics in grad school was that I didn’t enjoy engineering, and had previously performed very well in Mathematics in high school. Hence, my past performances reinforced my affinity for Mathematics, which led me to change my whole life trajectory. On the other hand, I was never very receptive to reinforcement by social acceptability, which has hampered my social behaviour for many years. Hence, receptivity to reinforcement is perhaps a factor that the authors have left out from the paper.

Different levels can be different

Consider alcohol consumption. Alcohol consumption is negatively correlated with cognitive abilities at an intra-person level, while being positively correlated with cognitive abilities at an inter-person level. How do we reconcile these two facts? These are completely different and non-contradictory facts. We are just saying that smart people tend to drink more alcohol than not so smart people (sorry teetotaling Hitler), and smart people get less smart when they drink a lot of alcohol.

This example illustrates the fact that correlations between two traits can be completely different at an intra-person and inter-person level. However, individual intra-person traits are still indicative of inter-person traits.

Dynamics within individuals

Consider the following quote:

Dynamic models imply more than the mere observation that people differ over time, for this could just be random fluctuations around a mean level.

This is a realization that I had myself a couple of weeks back! People don’t generally stay the same their whole life, their behaviour fluctuating around a stable mean. People change slowly but irrevocably with time (their mean behaviour changes with time). And it is around this changing mean that behaviour fluctuates.

We will now describe the mathematical model of dynamic personality developed by the authors. Let T be the desire or action tendency related to a particular action, F be the external conditions causing that desire, and c the “amount of action” taken to quench that desire. Then the rate of change of desire is dT=F-cT. In other words, your desire to do something will keep increasing with time unless you act upon it, which will then cause it to decrease sharply. If you desire to eat a chocolate cake, your desire to do so will keep increasing until you take a bite, which will cause it to decrease momentarily. Moreover, the decrease is proportional the desire itself!

When does dT become 0? This happens when F=cT, or in other words when you’ve done enough to quench that desire. Note that desire may not actually become 0 when dT=0. Hence, when the action c is removed, the desire again starts growing, until more action is taken. Effectively, enough action c has to be taken until T=0.

What is F, exactly? It is expressed as p_s(1-p_s)N_{ach}, where p_s is the probability of success and N_{ach} is the “need for achievement” (in other words, ambition). In other words, our desire to do something doesn’t necessarily increase with the ease with which it can be done. Desire is maximal when the probability of success is equal to the probability of failure. What does that mean? If I’m a naturally gifted athlete, and it is apparent that I am almost certain to win the Olympiad gold, I will not really want to pursue athletics that badly. However, if I’m a great musician but my chances of making it to the big stage are 50-50, I will be tempted to gamble on my life and see if I have what it takes to make it. This hit home for various reasons. As humans, we are not really maximizing our chances of having a comfortable life. We are always looking to gamble on our lives, and see if we have what it takes. That is why we are so open to new challenges and get excited by uncertainty (under the right circumstances). Although I probably have to think through some of the deeper implications of this, this equation did blow my mind!

There is an analogous model for negative tendencies or anxiety. Let N be the anxiety related to a particular task, I the inhibitory forces (factors that are causing anxiety), and r the cost of resisting action. Then we have dN=I-rN. In other words, anxiety about a pending homework assignment keeps rising (if the cost of doing homework is too low) or falling (if the cost of doing homework is too high) until it stabilizes. There is no action that you can perform which will make doing your homework less of an anxious experience.

What does this teach us? We all have tasks that we find distasteful but have to do from time to time. And waiting longer to do them is not going to make them any less distasteful (might only increase anxiety). Hence, whenever you have a task that you have to do, do it first. And then when it is over, you can get back to enjoying your life.

Again I is defined as p_s(1-p_s)N_{af}, where p_s is the probability of success and N_{af} is the need to avoid failure. We don’t really get anxious about things that have a low probability of success (asking someone out who will almost surely say no) or a high probability of success (asking someone out who will surely say yes). Our anxiety is the highest in situations which have a 50-50 chance of success (asking someone out just above our league).

Essentially, whether we do something or not depends on p_s(1-p_s)(N_{ach}-N_{af}). Things that have a 50-50 chance of success induce both the highest excitement and the highest anxiety in us. This says something very deep about human nature, which I’ll leave up to you to interpret.

The authors note that in this model of the human personality, there is no fixed or control point. There is no fixed value that positive or negative tendencies return to. In some sense, both desires and anxieties are unbounded.

Some more aspects of the model

You might want to eat pizza and go for a run at the same time. However, what you end up doing depends on what you want to do more badly. Hence, although desires for various actions grow in parallel, the actions themselves can only happen one after the other, and the action with a higher desire function is performed first. For instance, a newt copulates underwater, but comes up to the surface for oxygen. If oxygen content is increased, the chances of the newt being able to breathe successfully increase. Hence, the desire to go up on top decreases (remember that F=p_s(1-p_s)N_{ach}), and the newt copulates underwater for longer.

Another aspect of this is that because attention is limited, it can only be allotted to various tasks sequentially. Hence, one must not text while driving, watch tv while doing a cognitively intense task, etc.

Inter-person dynamics

The first observation that the authors make is that rates of change are more important than absolute levels. For instance, a person whose anxiety suddenly spikes is more likely to behave irrationally than someone who is always anxious.

Secondly, they state that the average levels of what one person does is different from the maximum levels that that person is capable of. For instance, if you work for 5 hours a day, that does not preclude the fact that you may be capable of 20 hours of work in a day when motivated.

Thirdly, the authors state that most cognitive abilities correlate with other cognitive abilities. A straight A student is probably also good at music and debating and all kinds of other activities that you may think of. This is perhaps the reason why top Business schools prefer that their candidates demonstrate excellence in multiple fields- a demonstrated expertise in multiple areas is of the best signals of overall cognitive abilities.

Cognitive abilities are sometimes described as the speed at which a solution to a problem is found/speed of processing. It has also been seen that the amount of crystallized knowledge is positively correlated with processing speed. Hence, smart people are more likely to be repositories of knowledge…at least in school (this may change later in life, when smart people may no longer be incentivized to accumulate irrelevant knowledge).

Now consider the following quote:

If temperament is what you usually do, and ability is what you can do, interests are what you like to do and how you spend your time.

One way to interpret this line is that abilities or the capacity for a certain kind of work does not necessarily form one’s temperament. Moreover, temperament may be different from interests. For instance, although you may have an angry temperament (reinforced perhaps by a lack of opposition from others), being angry might not be how you like spending your time. The authors also note that interests can be classified into people vs things and facts vs ideas.

Now let us consider the following quote:

That what one can do (ability or competence) is not necessarily shown by what one does has been known since at least Tolman and Honzik (1930) who studied the effect of reward on maze performance. With the same number of learning trials, non-rewarded rats take far longer to run a maze than when given a reward.

One way of interpreting this line is that if you want to assess a person’s maximal capabilities, attach a reward to the successful completion of a task. Only then will that person really exert themselves in attaining that goal.

Group differences as the consequence of individual choices

Imagine that there are two students- A and B. A feels slightly more anxious during public speaking than B. Hence, B generally does a better job at speeches than A. A receives negative reinforcement through audience disinterest or discouragement, while B receives positive reinforcement in the form of adulation. These effects compound, and A may join a profession that does not involve much public interaction while B may become a lawyer or a politician. Hence, individual differences compound and result in individuals joining different groups.

The natural question to ask at this point is what personality traits lead a student to choose a particular major, say Physics? In a large scale study, it was found that having high cognitive abilities and being less agreeable was highly correlated with choosing Physics (sorry Physicists). This is just a small example of how individual differences lead to group differences.


The authors conclude by clarifying that individual differences don’t deterministically lead to group differences. It is not as simple as that. Although individuals are blessed with their own abilities and desires, individuals change as a result of reinforcement from society or other external factors. “Personality needs to be conceived at multiple temporal durations.”

The authors also emphasize the importance of using softwares and formal methods to analyze data and create more accurate mathematical models.


  1. A model for personality at three levels, by Revelle and Condon

Published by -

Graduate student

2 thoughts on “Decoding human personality

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: