Beware of the blob

It creeps, and it might be more like us than we care to admit. That was a lesson I learned last fall when trying to choose between pigeons and slime moulds for our lab journal club. The birds, it seems, are on a different level.

It started with the Monty Hall problem and a new study that asks, “Are birds smarter than mathematicians?”1. For those not familiar, the Monty Hall problem is a puzzle made famous by columnist Marilyn vos Savant, based on the popular 1960s game show Let’s Make a Deal (which was, incidentally, hosted by Winnipeg-born Monty Hall). Here it is:

Suppose you’re on a game show, and you’re given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what’s behind the doors, opens another door, say No. 3, which has a goat. He then says to you, “Do you want to pick door No. 2?” Is it to your advantage to switch your choice?2

If you were on Let’s Make a Deal, would you take Hall’s offer to switch doors? Or would you stand by your original choice?

Let's Make a Deal

Does it make any difference?

Most would say that it does not matter what you do. The car is behind one of the two remaining doors, so the probability of winning is 1/2 no matter what. In her column, Marilyn vos Savant famously claimed otherwise: if you take the offer to switch, you can double you chance of winning from 1/3 to 2/3.

She was right, and here’s why: there are always three possible outcomes, even after one door gets eliminated. Maybe you picked the correct door at first, and switching would make you lose – but there are two ways that you could have been wrong with your first pick. In both of these scenarios, switching would give you the car. In other words, 2/3 of the time your first choice will be wrong, and accepting Hall’s offer will guarantee the win. The switch is only a mistake 1/3 of the time. If you still find this hard to believe, The New York Times has a good illustration.

Marilyn vos Savant’s explanation was so counter-intuitive that thousands of people wrote angry letters to her in disagreement, including a number of academics and mathematicians. It took a massive simulation experiment, with thousands of school teachers around the country running versions of the game in class, to convince people that she was correct. You can run your own simulation on the New York Times site as well.

It turns out that pigeons have a much easier time with the Monty Hall puzzle than we do. In a paper that came out last year, Walter Herbranson and Julia Schroeder trained pigeons and humans to play games based on the Monty Hall puzzle1. The apparatus they used for the pigeons is a standard one in animal psychology research: the birds were in a box with three different keys that they could press by pecking, and a mechanical feeder would deliver grain depending on their choices. The pigeons were trained to make two pecks analagous to the Monty Hall game. They made an initial choice out of three keys, followed by a second choice to either stay or switch after one key was removed as an option; this was communicated to the pigeons by changing the way the keys were lit up. Before every trial, the computer randomly chose one of the three keys for the prize. Pigeons who chose it on their second peck won a tasty reward: a grain pellet free-for-all that lasted three whole seconds.

Next, Herbranson and Schroeder set up a similar game for thirteen undergraduate students. All of the details of Let’s Make a Deal were stripped away, and what the students saw was as close to the pigeon version as possible: a touch screen monitor with three buttons. The students were told that they would earn points for their choices with the computer providing feedback, and that they should try to earn as many points as possible. That was it.

At the beginning, both the pigeons and the undergraduates chose more or less randomly. Within 30 days of training, the birds had learned to switch their choice most of the time. After hundreds of trials, the students were also switching more often than staying, but they never reached the switching level of the pigeons.

This is similar to results of earlier studies where humans were given a repeated version of the Monty Hall dilemma. They never really learn the game, and most people end up with a strategy of switching about half of the time3.

The reason we lag behind the pigeons, and why we find the Monty Hall problem so counter-intuitive, is that our complex brains rely on mental shortcuts most of the time. And the more advanced we get, the more we rely on these heuristics, ignoring evidence from observation as a result. For example, Herbranson and Schroeder point out that although adults and high-school students almost never pick up on the repeated Monty Hall game, some younger children get the switching strategy after seeing it play out several times in a row4. Children are more likely to learn from experience rather than blindly adhere to rule. Birds are masters of this. For instance, when Herbranson and Schroeder had pigeons play a modified version of the Monty Hall game where the payoffs for switching and staying were truly 50/50, each bird seemed to adopt its own unique strategy depending on what had happened to it in the first few trials.

Psychologists and marketing researchers can list close to 100 ways that we make predictable errors, due to our use of mental shortcuts. Think you might be different? You are probably just feeling the effects of your bias blind spot: our tendency to assume that we are less prone to these kinds of errors than other people. Here’s another one that might be familiar: the contrast effect. When we are choosing between multiple options, the presence of a third choice that contrasts the other two in some way will tend to amplify the importance of that particular characteristic.

The contrast effect explains why it is so profitable for stores to give us shelves full of similar products. You might prefer the cheap milk over the expensive organic version, but if you also see a third expensive-and-non-organic variety, there is a better chance you’ll shell out for the pricey organic milk.

Here’s where it gets weird: other organisms, even single-celled ones, do this too. In a recent paper, two researchers from the University of Sydney, Australia, showed that slime moulds also suffer from the contrast effect5. With some clever marketing you can manipulate the food choices of the blob.

Physarum polycephalum

A slime mould does its thing. Photo accompanies article.

Slime moulds are amoeboid organisms that look like, well, slime. They spend most of their time creeping along the forest floor eating microorganisms and dead material as they go. Tanya Latty and Madeleine Beekman used the vegetative stage of the common slime mould Physarum polycephalum for their study. Don’t let size fool you: although it’s on the order of inches, this blob is a single-celled organism with no nervous system. Physarum loves oatmeal but hates exposure to UV light, and Latty and Beekman took advantage of this in an experiment designed to pit these preferences against one another.

The researchers gave individual slime moulds the choice of three foraging sites where they manipulated the oatmeal concentration and the presence of UV light. The moulds could initially touch and assess all three food dishes; they would ultimately choose one by creeping, very slowly, in that direction. This took about 24 hours (watch a time-lapse video).

Predictably, the slime moulds tended to choose the best oatmeal option and avoid UV light. There was balance point in the trade-off, where the extra oatmeal was no longer worth the cost of light: 5% oatmeal under UV light is about as good as 3% eaten in the dark. But when the researchers also gave the moulds a third option of low oatmeal (1%) in the dark, they shifted their preference to 3% in the dark over 5% light. Provided they were not hungry, that is – starved moulds tended to stick with the higher concentration oatmeal no matter what.

The mere presence of a third option made the blob change its evaluation system. Like us, slime moulds must have some kind of comparative decision-making heuristic, but they do it without a nervous system. So how do they choose? Decision-making for a slime mould is a collective process. Different parts of the amoeboid cell sense their immediate surroundings, and “oscillate” at a specific frequency as a result6. In places where the cell touches preferred stimuli like oatmeal, high oscillation frequencies cause more of its biomass to flow in that direction. This is similar to the way ant colonies make democratic decisions about where to set up house7.

According to Latty and Beekman, it may be no accident that slime moulds have some of the same heuristics that we do. Computationally-speaking, the comparative evaluation method tends to save time and effort, whereas carefully weighing all of your options on all possible attributes tends to be much more of a chore. Heuristics can lead to predictable errors that shopkeepers exploit, but they are probably beneficial for humans and slime moulds alike when foraging in the real world.

In spite of their performance on the Monty Hall puzzle, birds are prone to comparative foraging errors too8,9. In the Monty Hall experiment, Herbranson and Schroeder noted that pigeons, like humans, start out with a slight bias towards staying with their initial choice. Also, when the experimenters gave the birds a reversed Monty Hall game where staying was the better strategy, they seemed to have an even easier time learning1. Biased decision-making is all around us, and Physarum proves that a complex brain is not required.


  1. Herbranson W. T. and Shroeder, J. 2010. Journal of Comparative Psychology 124: 1-13.
  2. Whitaker, C. F. “Ask Marilyn” in Parade Magazine, 9 September 1990.
  3. Granberg, D. and Brown, T. A. 1995. Personality and Social Psychology Bulletin 21: 711-723.
  4. DeNeys, W. 2006. In: Psychology of decision making in education. Nova.
  5. Latty, T. and Beekman, M. 2010. Proceedings of the Royal Society B 278: 307-312.
  6. Durham, A. C. and Ridgway, E. B. 1976. Journal of Cellular Biology 69: 218-223.
  7. Edwards, S. C. and Pratt, S. C. 2009. Proceedings of the Royal Society B 266: 3655-3661.
  8. Bateson, M. 2002. Animal Behaviour 64: 251-260.
  9. Bateson et al. 2002. Animal Behaviour 63: 587-596.


  1. Hi, Roz

    Very good article. I think your focus is probably closer to the original intent of the study than mine was — and you know the material better than I do. I’m interested in the amount of computation that goes on beneath the conscious surface of our minds — the minds of all animals really. As I mentioned, I am constantly amazed by the flying ability of birds — something so common we usually take it for granted — but tiny songbirds routinely perform maneuvers that would put our best pilots and computer systems to shame. (Bumblebees and other insects do too.) In the same way, people don’t realize what a complex problem something as “easy” as catching a ball is if you analyze what it takes to accomplish it. I think you’re right about the mental shortcuts — I was trying to get at something similar when I mentioned the problems that people have with drawing — where our abstract mental image of, say, a table gets in the way of being able to see and draw what’s actually in front of us. The abstraction is a shortcut, a way of clumping a category of things together and pulling out the relevant information, which is all we typically need in day-to-day operations.

    Your post is a really good look at the whole Monty Hall problem. I’ll put up a link to it on my blog if you don’t mind.

    Alex Washoe

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