Monday, March 11, 2013

Marshmallow Test

This is probably one of the best known experiments in psychology, thanks to the viral video below. You put a four year old in a room alone, present him with a marshmallow (or pretzel, or cookie, depending on the kid's preferences), and tell him that you are going to step out of the room and if the treat is still uneaten when you return, you will give them twice as many treats. The dependent variable here is how long the kid waits before eating the treat (the experimenter returned after 15 minutes; the kids were watched through a two-way observation mirror).

Here's the original research article.

Cute kids, right?

But the story takes a darker turn (as most good stories do).

Years later, the original researcher, Walter Mischel, tracked down the original participants and found that "teenagers who had waited longer for the marshmallows as preschoolers were more likely to score higher on the SAT, and their parents were more likely to rate them as having a greater ability to plan, handle stress, respond to reason, exhibit self-control in frustrating situations and concentrate without becoming distracted." [Source: APA]

"Recently, [researchers] tracked down 59 subjects, now in their 40s, who had participated in the marshmallow experiments as children. The researchers tested the subjects’ willpower strength with a laboratory task known to demonstrate self-control in adults.

Amazingly, the subjects’ willpower differences had largely held up over four decades. In general, children who were less successful at resisting the marshmallow all those years ago performed more poorly on the self-control task as adults.

Additionally, Casey and colleagues examined brain activity in some subjects using functional magnetic resonance imaging. When presented with tempting stimuli, individuals with low self-control showed brain patterns that differed from those with high self-control. The researchers found that the prefrontal cortex (a region that controls executive functions, such as making choices) was more active in subjects with higher self-control. And the ventral striatum (a region thought to process desires and rewards) showed boosted activity in those with lower self-control.

Research has yet to fully explain why some people are more sensitive to emotional triggers and temptations, and whether these patterns might be corrected."


"Amazingly"? Really? The American Psychological Association is "amazed" by these findings?

Adj. 1. amazed - filled with the emotional impact of overwhelming surprise or shockamazed - filled with the emotional impact of overwhelming surprise or shock; "an amazed audience gave the magician a standing ovation"; "I stood enthralled, astonished by the vastness and majesty of the cathedral"; "astounded viewers wept at the pictures from the Oklahoma City bombing"; "stood in stunned silence"; "stunned scientists found not one but at least three viruses"
surprised - taken unawares or suddenly and feeling wonder or astonishment; "surprised by her student's ingenuity"; "surprised that he remembered my name"; "a surprised expression"

Why wouldn't you expect the 4 year olds with higher willpower to have higher willpower as 40 year olds? We have known for over a century that kids who measure in the top percentiles of intelligence at age 5 are likely to be found in the top percentiles at age 15 and again at age 50. For decades we have known that fearful toddlers are more likely to grow up to be shy and inhibited adults. Why would this trait (call it "impulse control") be any different?

By the way, please note that it would not be surprising if the differential brain functioning observed in adulthood was also noted in childhood. In other words, the brains of the two sets of kids (high impulse control versus low impulse control) were already different from each other at age 4 years.

This is kind of amazing, I admit:

"The child who could wait fifteen minutes had an S.A.T. score that was, on average, two hundred and ten points higher than that of the kid who could wait only thirty seconds."

So, the marshmallow test is an IQ test? We're not sure -- we would need to have cognitive ability scores at age 4, which were not collected. If the 4 year olds' cognitive ability correlated very strongly to how long they delayed eating the treat, then we needn't talk about impulse control at age 4 predicting SAT scores, susbtance abuse, etc.. We already know that IQ predicts those things. The conceptual question is: Is impulse control a facet of general intelligence? The practical question is: Can impulse control be modified? (We know that IQ can't be modified, except in the wrong direction -- wear a helmet when you snowboard!)

Well, impulse control can be modified, by ingesting Ritalin, Adderall, Vyvanse, or any other ADHD medication. Can it be taught? We don't know.

Be careful when thinking about the "good parents" you know who "teach" their kids "self-control" through their parenting behaviors. A big part of why those kids end up with high self-control is because they are the biological offspring of people with high self-control.

Which brings us to this:

Anokhin, A.P., Golosheykin, S., Grant, J.D., & Heath, A.C. (2011). Heritability of delay discounting in adolescence: A longitudinal twin study. Behavioral Genetics, 41(2), 175-183.


Delay discounting (DD) refers to the preference for smaller immediate rewards over larger but delayed rewards, and is considered to be a distinct component of a broader “impulsivity” construct. Although greater propensity for discounting the value of delayed gratification has been associated with a range of problem behaviors and substance abuse, particularly in adolescents, the origins of individual differences in DD remain unclear. We examined genetic and environmental influences on a real-life behavioral measure of DD using a longitudinal twin design. Adolescent participants were asked to choose between a smaller ($7) reward available immediately and a larger ($10) reward to be received in 7 days. Biometrical genetic analysis using linear structural equation modeling showed significant heritability of DD at ages 12 and 14 (30 and 51%, respectively) and suggested that the same genetic factors influenced the trait at both ages. DD was significantly associated with symptoms of conduct disorder, attention deficit hyperactivity disorder, substance use, and with higher novelty seeking and poor self-regulation. This study provides the first evidence for heritability of DD in humans and suggests that DD can be a promising endophenotype for genetic studies of addiction and externalizing disorders.
"This study provides the first estimate of the heritability of DD in humans. Using a real-money choice paradigm, this study demonstrated that individual differences in the discounting of delayed gratification among adolescents are moderately to strongly influenced by genetic factors at ages 12 and 14 respectively. The results also suggest an increasing role of genetic factors with age. Given that DD is considered to be one of the key components of a broader impulsivity trait (; ), the demonstration of a significant genetic component in DD has important implications for genetic studies of psychopathological conditions characterized by increased impulsivity, particularly those involving impulsive decision-making, such as addictions, pathological gambling, externalizing disorders, and ADHD.


Taken together, the present findings and previous literature suggest that DD may represent a core neurocognitive dysfunction contributing to a range of problem behaviors characterized by reduced sensitivity to the delayed consequences of one’s decisions and actions. The preference for immediate but smaller rewards may be a behavioral marker of underlying genetic liability to impaired decision making and could serve as a useful endophenotype for genetic studies of the etiology of substance use disorders."

So those cute kids who just couldn't resist eating the marshmallow in the YouTube video? You're laughing at a neurocognitive dysfunction with a strong genetic basis. Here's what those kids look like as teenagers. And here's what they look like in their 40s.


Walter Mischel

This is an interesting article from the New Yorker about Walter Mischel and the marshmallow test. It mentions the various ways in which Mischel retarded the advance of personality science, e.g., declaring that personality traits couldn't be measured accurately, and that context was more important than personality.

From Wikipedia:

"In 1968, Mischel published the now classic monograph, Personality and Assessment, which created a paradigm crisis in personality psychology that changed the agenda of the field for decades. The book touched upon the problem in trait assessment that was identified by Allport back in 1937. Mischel showed that study after study failed to support the fundamental traditional assumption of personality theory, that an individual’s behavior with regard to a trait (e.g. conscientiousness, sociability) is highly consistent across diverse situations. Instead, Mischel's analyses revealed that the individual’s behavior, when closely examined, was highly dependent upon situational cues, rather than expressed consistently across diverse situations that differed in meaning.Mischel maintained that behavior is shaped largely by the exigencies of a given situation. That people act in consistent ways across different situations, reflecting an underlying consistency of personality traits, is a myth."

The only problem with that view is that is it wrong, insofar as it is usually interpreted to mean that the measurement of personality traits cannot yield accurate predictions of future behavior. It is ironic that Mischel is famous for two things: 1) saying that personality isn't consistent and traits don't predict future behavior; and 2) developing a personality test (the marshmallow test) that predicts future behavior (SAT scores, ADHD, substance abuse) and appears to assess a stable trait that is consistently demonstrated across a wide variety of situations (e.g., school, home, work).

There is mention in the New Yorker article about work by Mischel and Angela Duckworth that proposes to increase impulse control. My guess is that these will turn out about as well as prior attempts to improve IQ.


Finally, here's a summary of another interesting study that uses the marshmallow paradigm. In this one, the experimenters either break or fulfill a promise made to the kids. Whether the kids eat the marshmallow depends on whether the earlier promise had been broken or fulfilled. The researcher suggests that the reason poor kids eat the marshmallow (i.e., display low self-control) is that they have a history of broken promises and therefore less reason to believe the promises of adult experimenters. It's not because they are less intelligent or have genetically-mediated neurocognitive dysfunction.

Well, if Walter Mischel can start collaborating with behavioral geneticists, I can certainly tip my hat to the influences of early childhood environment. But you also have to acknowledge that promise breakers probably have low impulse control, and impulse control is at least partially genetic, so these kids are getting a double dose of vulnerability from their parents: the parents pass along a vulnerability to impulse control disorders, and provide a less-than-ideal environment that interacts with that genetic vulnerability.

Where people go wrong is when they neglect either side of the Genotype x Environment = Phenotype interaction. So, both the "broken promises" study and the twins study are interesting and important, but we have to integrate them in order to understand what's really going on.





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