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Category: Effects
Type: A sense that one entity, within a reasonable range of sizes, is the appropriate amount to engage, consume, or consider
Origin: Andrew B. Geier, Paul Rozin, and Gheorghe Doros, Psychological Science 17(6), June 2006, pages 521–525
Also known as: A one-unit portion heuristic
Quick Answer — Unit Bias is a sense that one entity, within a reasonable range of sizes, is the appropriate amount to take. Andrew B. Geier, Paul Rozin, and Gheorghe Doros named it in 2006. In a lobby, whole pretzels and half pretzels were free, and people could take more than one. The mean ratio of daily selected weight, large day over the paired small day, was 1.69. A pure unit account would have predicted 2.00. A later cookie study is a different object. Women there ate more than one piece.

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What is Unit Bias?

Unit Bias is a sense that a single entity, within a reasonable range of sizes, is the appropriate amount to engage, consume, or consider.
Unit bias is a sense that a single entity (within a reasonable range of sizes) is the appropriate amount to engage, consume, or consider.
That sentence is Geier, Rozin, and Doros in 2006. The entity can be a piece of food, a scoop, or, in their discussion, a pill. “One” names the entity. It does not name the weight. A tablespoon and a quarter-cup scoop can both be called one spoon, and the quarter-cup scoop is four times as large. A teacup and a soup bowl can both be called one cup. The word did not change. The rice did. The authors call Unit Bias a heuristic, and they offer it as one account of why a larger portion is followed by a larger intake. The three studies on this page measured what left a free bowl. They did not measure a bite. Default Effect is a preselected option that stays in place. Denomination Effect is the face value of the note in hand. A 12-gram candy and a 3-gram candy are a piece size. They are not two notes, and they are not a box already checked.

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Unit Bias in 3 Depths

  • Beginner: One piece can be 3 grams or 12 grams. Read the weight before you call the piece a serving.
  • Practitioner: When the piece or the spoon changes, write the weight that leaves, not only the count of pieces. The 1.69 is paired lobby days.
  • Advanced: The ratios sit between equal weight and pure unit control. A later cookie study found intake above one piece. Neither figure is a household’s diet.

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Origin

The name is Geier, Rozin, and Doros’s. The paper is “Unit Bias: A New Heuristic That Helps Explain the Effect of Portion Size on Food Intake,” Psychological Science, volume 17, number 6 (June 2006), pages 521–525. All three authors are listed at the University of Pennsylvania. The snacks were free, in large numbers, in public. Two studies changed the piece. One study changed the spoon. Geier uses a supermarket yogurt size as an illustration of a smaller unit, citing Paul Rozin, Kimberly Kabnick, Erin Pete, Claude Fischler, and Christy Shields, Psychological Science, volume 14 (2003), pages 450–454. He prints a modal individual portion of 125 grams in French supermarkets and 227 grams in American supermarkets. This page did not re-read that 2003 paper. Geier does not report a count of shoppers who bought a second cup. The yogurt pair is his example of unit size. It is not one of the three snack studies. The boundary study is Katerina Kerameas, Lenny R. Vartanian, C. Peter Herman, and Janet Polivy, “The Effect of Portion Size and Unit Size on Food Intake: Unit Bias or Segmentation Effect?,” Health Psychology, volume 34, number 6 (June 2015), pages 670–676. They argue that eating less from smaller pieces is better called a segmentation effect. The journal PDF was not re-read. The portion sizes below are a public HTML text of the article. The result sentences are the abstract. No mean intake from that paper is on this page.

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Key Points

Four readings keep each ratio on the bowl that produced it. A day’s leftovers are not a person’s plate. A cookie eaten in a later study is not this lobby.
1

Call it one only inside a size range

The authors say the alternate sizes have to fall among acceptable units. A single jelly bean, or a single M&M, is their example of a piece too small to be a functional unit. A piece larger than an acceptable portion is their example of a unit that has been exceeded. The Tootsie Roll pair they actually served was 3 grams and 12 grams. The spoon pair was a tablespoon and a quarter-cup scoop, which they call four times as large. Those pairs are the test. The jelly bean is the boundary they state, and it has no count on this page.
2

Hold the starting weight and change the piece

Each Tootsie Roll morning began with 240 grams, either 80 small pieces or 20 large pieces. The piece changed by a factor of 4. The bowl’s starting weight did not. Pretzels were 60 whole pieces, at 3 ounces each, or 120 halves, at 1.5 ounces each. People could take more than one, and the snack was free. A regulation story in which only calories matter would predict the same weight either way. A pure unit story would predict the same number of entities, so the large-piece days would carry 4 times the Tootsie Roll weight and 2 times the pretzel weight.
3

Read a ratio of paired days

The datum is the weight selected on a day, from a count of leftovers. Tootsie Roll days and M&M days each produced five ratios, one for each weekday. Pretzel days produced 18 ratios, pairing the first Monday with the second Monday, and so on. The ratio is scaled so that 1.00 means the two days selected the same weight. The Tootsie Roll mean was 2.27, against a pure-unit prediction of 4.00. The pretzel mean was 1.69, against 2.00. The M&M mean was 1.67, against 4.00. These are means of day-pairs. They are not means of people.
4

Stop short of pure unit control

Every pretzel ratio was above 1.00, 18 of 18. Every Tootsie Roll ratio and every M&M ratio was above 1.00, 5 of 5. The large piece raised the daily weight. It did not raise that weight by the full factor of the piece. The authors say the number of large entities taken did not equal the number of small entities taken, so other factors also influence selection. The tests are one-tailed. Details sit in the case. A sign that says to eat your fill is a wording. Framing Effect is a different wording result. The M&M sign did not pull the two scoops to the same daily weight.

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Applications

Use the piece weight, the day’s leftovers, and the spoon. Keep each ratio inside the bowl that produced it.

Write the grams beside the word one

Before you treat one piece as the serving, write what the piece weighs. The Tootsie Roll bowl started at 240 grams either way. The small piece was 3 grams, and the large piece was 12 grams. The word one covered both. The list of grams is the check. The five day-pairs are that office bowl.

Count pieces and weigh what leaves

Cutting a pretzel in half doubles the count in the lobby, from 60 to 120, and halves the piece, from 3 ounces to 1.5 ounces. Write both numbers, then write the weight gone at the end of the day. The mean ratio of those paired days was 1.69. A pure unit account predicted 2.00. The 1.69 is 18 day-pairs in one alcove.

Change the spoon and leave the food free

The M&M bowl held 1 pound, with a spoon chained to it. One spoon was a tablespoon. The other was a quarter-cup scoop, four times as large. The sign said to eat your fill and to use the spoon. People could take more than one spoonful. The mean ratio of paired days was 1.67, against a pure-unit prediction of 4.00. The 1.67 is five day-pairs at one desk.

Ask whether the piece is a unit at all

A single M&M is the authors’ example of a piece too small to count as a unit. A piece past an acceptable portion is their example of a unit already exceeded. Use that range as a question about the piece in front of you. The jelly bean has no count here. The lobby ratios are not a serving guide for a kitchen.

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Case Study

Andrew B. Geier, Paul Rozin, and Gheorghe Doros left free snacks in public and changed the unit. They worked at the University of Pennsylvania. The paper appeared in June 2006. The question was whether a larger unit would raise the weight people selected, when more units were available, the snack was free, and the extra effort of taking several small units was trivial. The Tootsie Rolls sat in a large mixing bowl on the first floor of an office building, in a high-traffic spot where snacks were already customary. During the study this was the only snack there. On alternating days the bowl held 80 small rolls, 3 grams each, or 20 large rolls, 12 grams each. Each morning started at 240 grams. At the close of the day the remaining pieces were counted. That count ran for 10 working days, five for each size. The pretzels were Philadelphia-style soft pretzels in an upscale apartment building. Management already left them for tenants on Mondays, Wednesdays, and Fridays, in an alcove off the lobby. They were normally whole, at 3 ounces. On alternating weeks the researchers left 60 whole pretzels or 120 halves, at 1.5 ounces. The pattern ran 12 weeks, which gave 18 days for each size. Pretzels went out at 8:00 a.m. Leftovers were counted at 4:00 p.m. The building is unnamed. Philadelphia-style names the pretzel. It does not name the city of the lobby. The M&M’s used the same apartment building in a different period. A bowl of 1 pound sat on the concierge’s front desk. A tablespoon or a quarter-cup scoop was chained to the bowl. The quarter-cup scoop is four times the tablespoon. A sign read “Eat Your Fill,” with “please use the spoon to serve yourself” in smaller type. Service ran from 9:00 a.m. to 5:00 p.m. on 10 working days, Monday through Friday, alternating the scoop. The basic datum is weight selected per day. The paper’s figure plots that daily weight. This page does not read grams off the figure. People could overlap from day to day, by an amount the authors call undetermined, so the days are not independent. The authors state that who approached did not differ by unit size. They do not print a count of people. They also compared the spread of the days. There were 28 small-unit days, the five, the five, and the 18. In the Tootsie Roll days and in the M&M days, the highest small-unit day did not rise above the lowest large-unit day. In the pretzel days, it did. They give the probability of that pattern by chance as .004 for the Tootsie Rolls, .004 for the M&M’s, and below .001 for the pretzels. They call this the prediction of a pure regulation account, in which unit size would not matter. They do not name the test beyond that probability. The ratio tests are one-tailed, against 1.00. For the Tootsie Rolls the mean ratio was 2.27, t(4) = 1.92, p = .064, d = 0.86, and all five ratios were above 1.00, binomial p = .031. The t test did not reach the .05 line. The high variance came from one ratio of 4.8. The authors say that dropping this strongest ratio made the t test significant. They do not print the new statistic. For the pretzels the mean was 1.69, t(17) = 10.21, p below .001, d = 2.41, and all 18 ratios were above 1.00, binomial p below .001. For the M&M’s the mean was 1.67, t(4) = 7.56, p = .001, d = 3.38, and all five ratios were above 1.00, binomial p = .031. The lesson is three free bowls, a piece or a spoon that changed, and daily weights that rose by less than the piece itself. The 1.69 is 18 paired pretzel days. It is not a person who ate 1.69 pretzels. What they measured was selection. They assume most people ate what they took, on the spot or while walking away. Food may have been thrown away, given away, or pocketed. They think pocketing is unlikely for pretzels and M&M’s, and more plausible for wrapped Tootsie Rolls. Informal observation suggested prompt eating. They print no count of pieces pocketed. In the discussion they also offer a pill advertisement, a long movie, and an amusement-park ride as places the same sense might apply. Those sentences have no counts. They are not this lobby.

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Boundaries and Failure Modes

Unit Bias is the wrong name in three retellings. A day’s ratio is treated as one person’s plate. A later cookie study is treated as the same bowl. A face value, or a box already checked, is given this page’s ratios. The first boundary is the design. The days overlap by an unknown number of people. The tests are one-tailed. The Tootsie Roll t test sits at .064, while the binomial on the same five ratios sits at .031. The authors’ later sentence groups all three foods as significant effects in the predicted direction. The t test they print for the Tootsie Rolls does not itself clear .05. No head count of passersby is on the page. The 1.69, the 1.67, and the 2.27 stay on their day-pairs. A kitchen that copies the half pretzel has not copied the alcove. The second boundary is the cookie study. Kerameas and her coauthors served women a portion and varied whether it was one piece or three. Study 1 was 87 women, a 30-gram portion or a 90-gram portion, as one wrapped cookie or as three wrapped cookies of 10 grams or 30 grams. Study 2 was 42 women, 60 grams, as one cookie or as three 20-gram cookies on separate plates, unwrapped. The abstract says there was no evidence that participants prefer to eat a single unit. Several smaller pieces led to less intake than one larger piece, even when the total portion was the same, and the amount eaten was consistently more than a single unit. They say a perceived norm of appropriate intake carried the effect of unit number. Their conclusion is a segmentation effect, and portion size in these studies moved intake apart from how the portion was cut. The journal PDF was not re-read, so no mean and no p value from it is printed here. The 87 and the 42 are women in those two studies. They are not the lobby, and they are not a rate for men. The third boundary is a neighboring page. Anchoring Effect is a starting number that pulls a later judgment. The 12-gram roll is a piece in a bowl. It is not that starting-number study. Mental Accounting is a label on money by source or by use. The label on this page is the piece. The authors also say a culture may teach people to finish a unit, and that food in the hand, or a piece already started, can push in the same direction. That is their discussion. It is not a measured share of people who cleaned a plate.

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Common Misconceptions

Three retellings travel with the name. One piece is treated as a stopping rule with no exceptions. A ratio of days is treated as a person’s intake. A note’s face value, or a default already marked, is filed here.
A pure unit account predicted a weight ratio of 4.00 for the Tootsie Rolls and the M&M’s, and 2.00 for the pretzels. The means were 2.27, 1.67, and 1.69. The authors say the count of large pieces did not equal the count of small pieces. In the 2015 cookie studies, the amount eaten was consistently more than one unit, and the authors report no evidence of a preference for a single unit. One piece is the hypothesis they tested. It is not a rule the bowls confirmed without remainder.
The 1.69 is the mean of 18 ratios of daily pretzel weight, a whole-pretzel day over a paired half-pretzel day. The people overlap across days by an unknown amount. The scale was leftovers, not a weighed bite. Pocketing was more plausible for the wrapped Tootsie Rolls than for the pretzels. A person who takes one whole pretzel home has not reproduced the 18 day-pairs.
Denomination Effect is spending that depends on the face value of the note in hand. Default Effect is a preselected option that stays unless someone changes it. Unit Bias is the piece or the spoon treated as the right amount, inside a range of sizes. The lobby ratios do not move onto a banknote or onto a form.
Unit Bias sits beside a preselected option, beside the face value of a note, beside a starting number, beside a label on money, and beside the wording of a question.

Default Effect

A preselected option that stays in place. The piece in the bowl is a unit someone can take more of. It is not a box already marked.

Denomination Effect

Spending that depends on the face value of the note in hand. A 3-gram piece and a 12-gram piece are a candy size.

Anchoring Effect

A starting number that pulls a later judgment. The daily weights here are leftovers from a free bowl, not that number study.

Mental Accounting

A label by source or by use, judged inside that label. The split on this page is the piece, between the word one and the weight.

Framing Effect

The same facts, worded differently, draw different choices. The M&M sign said to eat your fill. The two scoops still left different daily weights.

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One-Line Takeaway

Before you treat one piece as the right amount, write what that piece weighs, and write what leaves the bowl when the piece is smaller.