influence of prior knowledge on recall for height
DESCRIPTION
This was presented at the Stanford Undergraduate Psychology Conference, 2011. Project completed with the help of Dr. Pernille Hemmer of Syracuse University and Dr. Mark Steyvers of UC Irvine.TRANSCRIPT
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The Influence of Prior Knowledge on Recall for Height
Jenny ShiPernille Hemmer
Mark Steyvers
Department of Cognitive ScienceUniversity of California, Irvine
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Introduction
Memory is not an exact replica of an event, but rather more of a noisy representation. We fill in the gaps to make sense of the memory. This “fill-in” is influenced by our expectations and prior knowledge.
People usually depend on this prior knowledge to draw conclusions, recall certain events, make decisions, solve problems
• Expectations, stereotypes, common sense
...but does prior knowledge help or harm our recall from memory?
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Prior knowledge harms
Much of previous literature has focused on negative impacts of expectations.
Participants falsely recalled books in an office setting, possibly because most offices are expected to contain books.
(Brewer & Treyens, 1981)
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Prior knowledge harms
Semantic word lists elicit false recall of words.
(Roediger & McDermott, 1995)
These studies show that expectations are detrimental to recall from memory because they induce false memories.
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BEDSNOOZEDREAMWAKERESTDOZEYAWN
SLEEP
Directions:
Recall words from the list
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Prior knowledge helps
Memory task with vegetables, fruits, and artificial shapes.
Reconstruction from memory was better for objects that subjects had pre-experimental prior knowledge for compared to objects they had no prior knowledge for.
(Hemmer & Steyvers, 2008)
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Current Study
Extension of Hemmer & Steyvers (2008)
Efficiency in analysis
• Hemmer & Steyvers used the prior knowledge of others as their true value. The current study uses the actual true value and is independent from subjectivity.
Moving towards naturalistic stimuli
• While the sizes of fruits and vegetables can vary across culture and time, human height stays relatively constant. The average height of an American male isn’t much different from an average European male.
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Why use height as prior knowledge?
Prominent difference between the mean height of males and females
• Different distributions and means between two genders
Objective scale of measurement
Consistency in nature
• Stereotype is drawn from real-world distributions
• Accurate and universal across societies
The concept is deep-seated
• In a study, subjects still used this prior knowledge even though they were told that gender and height had no correlation (Nelson, Biernat, & Manis, 1990).
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Absolute Height
Relative height of males
Relative heightof females
Overall distribution of heights
A
B
We predict…
A. Regression to mean• People are estimated towards the mean.
• People shorter than the mean are overestimated while people taller than the mean are underestimated.
B. Hierarchical effect• If prior knowledge is more fine-
grained, people are estimated towards the mean of their specific category.
• If tall female and short male presented at same height, the female will be underestimated while the male will be overestimated.
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Stimuli Production
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• Photographed total of 200 males and females with digital camera
• Height recorded in inches
• Stimuli height corresponded to true population
Height (inches)
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Stimuli Production
Unmasked stimuli – Retained all aspects of the figure, essentially revealing prior knowledge of gender
• 48 images were selected for this category: 24 ♂, 24 ♀
Masked stimuli – Retained only height and human figure info, restricting prior knowledge of gender
• 24 stimuli selected
• Silhouette of unmasked stimuli
• Gender ambiguity ensured through a ratings panel evaluation
Unmasked Male Unmasked Female
10Masked
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Experiments
Two separate experiments (Between-subjects)
Prior knowledge task
• Assessment how subjects’ prior knowledge affect height estimations
• Participants asked to judge how tall a person is based on whether prior knowledge of gender is restricted or revealed.
Memory task
• Assessment of how prior knowledge affects memory
• Continuous recall paradigm: Study and test phases are randomly intertwined
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Prior knowledge task
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What do you think is the true height of this person, compared to the door on the left?
OK
Reference image of a door
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Experimental Design for Memory Task
2 blocks with the memory task
• Block 1: 48 unmasked stimuli (24 male, 24 female)
• Block 2: 24 masked stimuli
Each stimuli was used for one trial (72 total)• Counterbalanced between and within blocks
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Memory Study – Study phase
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Memory Study – Test phase
What was the height of this person, compared to the door on the left, when you saw them at study? If you
are not sure, make a best guess.
OK
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Memory Task: General Results
A linear regression model fit to the data• Slope (regression to the mean)
• Intercept (hierarchical effect)
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Memory Task: Unmasked Results
Biased towards the mean height• Negative slopes indicate that
short people are overestimated and tall people underestimated
Each gender is biased towards their own specific height distribution
• Females estimated to be shorter than males
Directional biases based on gender info• When presented in same height,
females estimated shorter, while males estimated taller
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Memory Task: Unmasked Results
Biased towards the mean height • Negative slopes indicate that
short people are overestimated and tall people underestimated
Each gender is biased towards their own specific height distribution
• Females estimated to be shorter than males
Directional biases based on gender info• When presented in same height,
females estimated shorter, while males estimated taller
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Memory Task: Masked Results
Bias corresponds to the general distribution of human height
Our shortest stimuli was recalled with near perfect accuracy
• Possible outlier effect
• Could be attributed to a cue other than gender that’s inferring a shorter stature
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Memory Task: Masked Results
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Our shortest stimuli was recalled as short on average.
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Task Type Results
Recall error was greater for masked stimuli than unmasked.
• Restriction of gender information introduces greater bias.
Less error for Memory task compared to the Prior knowledge task.
Even when participants have not studied an image, performance is better for unmasked compared to masked stimuli.
• Prior knowledge is helpful
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Conclusion
By using naturalistic stimuli, we were able to obtain results that suggest having prior knowledge improves reconstruction from memory.
Influence of prior knowledge is hierarchical.
• General prior knowledge will create bias towards mean of general distribution.
• Access to finer-grained prior knowledge will differentially affect the memory for two people (male and female) originally presented at the same height. They will be biased towards the mean of their own specific distribution.
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Acknowledgments
Thank you to…
Pernille Hemmer
Mark Steyvers
Michael D. Lee
Memory and Decision-Making Lab, UCI
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Extra slides
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Gender Height Distributions
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Height in Inches
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Stimuli Production
STIMULI MEAN
TRUE MEAN
MALES 68.9 69.4†
FEMALES 63.9 63.8†
† = Figures obtained from McDowell et al. 2008. Figures reflect the U.S. population over age 20.
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Gender mean heightOur data:
Mean height Female: 63.9 inches (SE 0.24)
Mean Height Male: 68.9 inches (SE 0.36)
In US population (20-29 years old):
Mean height Female: 64.5 inches
Mean height Male: 70 inches
(US census: http://www.allcountries.org/uscensus/)
In US population over 20:
Mean height Female: 63.8 inches (SE 0.06)
Mean height Male: 69.4 inches (SE 0.07)
(CDC: http://www.cdc.gov/nchs/data/nhsr/nhsr010) 27
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Norming masked
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Correlation between estimated and true height of masked females and males. Both means for the genders are the same (about 155 cm).
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Norming unmasked
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Correlation between estimated and true height of unmasked females and males. Means for the genders are the different (female 140 cm; male 150 cm).
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Norming unmasked (female)
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Norming unmasked (male)
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Potential extensions...
Labeling• Show masked but also include a label of either “Male”,
“Female” or “Unknown”
• Can control the specific prior knowledge subjects receive.
• Differentially recall height based on the gender info provided
Outlier Effect• We didn't see one in the current study.
• Main problem was the stimuli.
• We need people with extreme heights (esp. males)
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