ral
ssBioMed Cent
Nutrition Journal
Open Acce
Research
Effects of social approval bias on self-reported fruit and vegetable
consumption: a randomized controlled trial
Tracy M Miller
1
, Madiha F Abdel-Maksoud
1
, Lori A Crane
1
, Al C Marcus
2
and
Tim E Byers*
1
Address:
1
Department of Preventive Medicine and Biometrics, University of Colorado Denver, 4200 East 9th Avenue, Denver, CO, 80262, USA
and
2
AMC Cancer Research Center, 1600 Pierce St., Denver, CO, 80214, USA
Email: Tracy M Miller - tracymarie.miller@state.co.us; Madiha F Abdel-Maksoud - madiha.abdel-maksoud@uchsc.edu;
Lori A Crane - lori.crane@uchsc.edu; Al C Marcus - Al.Marcus@uchsc.edu; Tim E Byers* - tim.byers@uchsc.edu
* Corresponding author
Abstract
Background: Self-reports of dietary intake in the context of nutrition intervention research can
be biased by the tendency of respondents to answer consistent with expected norms (social
approval bias). The objective of this study was to assess the potential influence of social approval
bias on self-reports of fruit and vegetable intake obtained using both food frequency questionnaire
(FFQ) and 24-hour recall methods.
Methods: A randomized blinded trial compared reported fruit and vegetable intake among
subjects exposed to a potentially biasing prompt to that from control subjects. Subjects included
163 women residing in Colorado between 35 and 65 years of age who were randomly selected and
recruited by telephone to complete what they were told would be a future telephone survey about
health. Randomly half of the subjects then received a letter prior to the interview describing this
as a study of fruit and vegetable intake. The letter included a brief statement of the benefits of fruits
and vegetables, a 5-A-Day sticker, and a 5-a-Day refrigerator magnet. The remainder received the
same letter, but describing the study purpose only as a more general nutrition survey, with neither
the fruit and vegetable message nor the 5-A-Day materials. Subjects were then interviewed on the
telephone within 10 days following the letters using an eight-item FFQ and a limited 24-hour recall
to estimate fruit and vegetable intake. All interviewers were blinded to the treatment condition.
Results: By the FFQ method, subjects who viewed the potentially biasing prompts reported
consuming more fruits and vegetables than did control subjects (5.2 vs. 3.7 servings per day, p <
0.001). By the 24-hour recall method, 61% of the intervention group but only 32% of the control
reported eating fruits and vegetables on 3 or more occasions the prior day (p = 0.002). These
associations were independent of age, race/ethnicity, education level, self-perceived health status,
and time since last medical check-up.
Conclusion: Self-reports of fruit and vegetable intake using either a food frequency questionnaire
or a limited 24-hour recall are both susceptible to substantial social approval bias. Valid assessments
of intervention effects in nutritional intervention trials may require objective measures of dietary
Published: 27 June 2008
Nutrition Journal 2008, 7:118 doi:10.1186/1475-2891-7-18
Received: 14 November 2007
Accepted: 27 June 2008
This article is available from: http://www.nutritionj.com/content/7/1/18
© 2008 Miller et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Page 1 of 7
(page number not for citation purposes)
change.

Nutrition Journal 2008, 7:18 http://www.nutritionj.com/content/7/1/18
Background
Dietary assessment methods based on self-reports are
widely used in nutritional intervention studies. Conclu-
sions drawn from these studies may be impeded, how-
ever, by the presence of social desirability (the tendency to
respond in such a way as to avoid criticism) and social
approval (the tendency to seek praise) biases, which are
tendencies for an individual to respond in a manner con-
sistent with expected norms [1-3].
Previous dietary studies suggest that survey respondents
who exhibit social desirability characteristics are more
likely to underreport energy and fat intake [2-6]. Only
limited work has been done to determine whether an
intervention or knowledge of the goals of an intervention
can produce biases using dietary assessment tools com-
mon in nutritional epidemiology. Kristal et al, using a
brief food frequency measure, elicited significantly lower
reports of fat intake from undergraduate students who
had been exposed to a brief nutritional message about fats
and health than from students not exposed to that mes-
sage [6].
Cognitive psychologists term the different types of mem-
ory required for dietary recalls vs. food frequency ques-
tionnaires (FFQs) as specific vs. generic. Specific memory
relies on particular memories about episodes of eating
and drinking, as in a 24-hour recall. Generic memory
relies on general impressions about one's typical diet, as
in a FFQ that directs a respondent to report impressions
about usual frequency of eating a food over the previous
year. As the time between the behavior and the report
increases, respondents rely more on generic memory and
less on specific memory General memory may be more
subject to social approval bias than are specific recalls of
actual recent events [7].
Several studies suggest that women's responses to dietary
intake questions may be more affected by social desirabil-
ity bias than those of men [2,3,6]. Hebert et al observed a
large underestimate of fat and energy intake associated
with increased social desirability score in a 7-day diet
recall (cognitively similar to the food frequency question-
naire), when compared with a 24-hour diet recall. For
total energy, this bias was approximately twice as large for
women as for men [2]. In a later study, Hebert et al found
that energy and fat intake were overestimated by men but
underestimated by women who had higher levels of social
desirability [3]. Kristal et al reported a similar finding in
his study of undergraduate students: underreporting of fat
intake from a food frequency questionnaire by women,
but not by men [6].
etable intake obtained from a short food frequency ques-
tionnaire, as well as from a limited 24-hour recall.
Specifically, we hypothesized that a group of women who
believed that the study intent was to measure the intake of
(healthy) fruits and vegetables might report higher intakes
of fruits and vegetables than subjects in a control group,
who would believe that the study had a more general pur-
pose. We also hypothesized that responses to the food fre-
quency questions might be more biased than responses to
the 24-hour recall.
Methods
Design
This study was carried out as a blinded randomized con-
trolled trial. All subjects were recruited by telephone
before random assignment to study groups. At the time of
recruitment, subjects were asked if they would complete a
short phone interview about food intake and health at a
future date for a study of eating habits of women living in
Colorado. Upon verbal consent to participate, each sub-
ject was then told she would soon receive a letter in the
mail followed by a telephone interview within ten days.
No specific appointments were made regarding the date
or time of the telephone interview. Each subject was
instructed to read the letter and have it accessible during
the interview.
Approximately 2–3 days after recruitment, each subject
received a letter in the mail reminding her of the upcom-
ing phone interview and providing information about
serving size definitions of foods. Prior to that letter, sub-
jects were randomized 1:1 to the intervention or control
group. Letters received by the intervention group were
printed on letterhead with colorful graphics of fruits and
vegetables and included a small amount of text prompt-
ing participants about the health benefits of fruits and
vegetables and recommendations to consume 5–9 serv-
ings of fruits and vegetables per day ("Colorful fruits and
vegetables provide a wide variety of vitamins, minerals,
fiber, and phytochemicals your body uses to maintain
good health and energy levels, protect against the effects
of aging, and reduce the risk of cancer and heart disease.
Americans should consume 5–9 servings of fruits and veg-
etables daily. Eating more fruits and vegetables may be
one of the easiest things you can do to improve your
health."). Intervention subjects also received a 5-A-Day
refrigerator magnet and a 5-A-Day sticker was placed on
the outside of the envelope. Letters received by the control
group were identical to those received by the intervention
group except that they were printed on black-and-white
University of Colorado Health Sciences Center letterhead,
and they did not include the above text about fruits and
vegetables, the 5-a-Day the sticker, or the 5-A-Day magnet.Page 2 of 7
(page number not for citation purposes)
In this study, we sought to estimate the magnitude and
direction of social approval bias in reported fruit and veg-

Nutrition Journal 2008, 7:18 http://www.nutritionj.com/content/7/1/18
Subjects
Women residing in the State of Colorado were randomly
selected for recruitment from a commercial database of
listed telephone households, specifically targeting
females, aged 35–64 (Survey Sampling, International).
This commercial database is drawn from telephone direc-
tories, school registration lists, magazine subscription
lists, voter registration lists, and driver's license lists.
Recruitment phone call attempts were made to 663 ran-
domly selected households from this list. Of the phone
numbers attempted, 26% were unreachable after seven
attempts and 13% of the phone listings were non-working
numbers or the intended person did not reside in the
household. One percent of the women contacted were
ineligible for the study according to their age or did not
speak English. Among the 396 women who were con-
tacted and eligible for the study, 206 (52%) agreed to take
part. After recruitment, we were unable to contact 34
women (18 controls, 16 intervention subjects) for the die-
tary assessment. Five women refused at the time of the
interview (2 controls, 3 intervention subjects) and 4 were
lost to follow up for other various reasons (1 control, 3
intervention subjects). Therefore, 163 women completed
the follow-up phone interview (83 controls, 80 interven-
tion subjects).
Phone interviews were typically completed within ten
days after the subjects received the letter. The interviews
were not scheduled. The phone interviews completed in
this study were conducted by trained, professional inter-
viewers, calling at random times, with the interviewer
being blinded as to which group each subject was
assigned. The phone interview consisted of a standard
introduction and preparation. Each subject then
responded to an 8-item food frequency questionnaire, a
brief 24-hour recall specific to fruits and vegetables,
demographic questions, questions regarding health sta-
tus, and a question about personal beliefs regarding the
relationship between fruit and vegetable intake and dis-
ease modelled after Trudeau et al [8]. The protocol for this
study was reviewed and approved by the Colorado Multi-
ple Institutional Review Board.
Food frequency assessment
Eight questions from the Behavioral Risk Factor Surveil-
lance System (BRFSS) were used to assess fruit and vegeta-
ble intake [9]. In addition, we included the BRFSS
questions regarding intake of fluid milk and sweet bread
products (doughnuts, cookies, cakes, pastries, and pies),
which were not mentioned in the biasing prompt, and are
generally considered neutral (milk) or socially undesira-
ble (sweets) to determine whether any bias we might
observe would be specific to fruits and vegetables. Sub-
reports, the total number of servings per day of fruits and
vegetables was determined.
24-hour recall
A brief version of the 2001 California Dietary Practices
Survey was used to assess fruit and vegetable intake on the
previous day [10]. This recall was a series of eight ques-
tions regarding whether the respondent ate each of three
meals or a snack the previous day, and (if eaten) whether
fruits and/or vegetables were consumed at each of those
occasions.
Subjects were also asked questions from the Behavioral
Risk Factor Surveillance System interview [11] about fac-
tors shown by others to be related to fruit and vegetable
intake. These included age, race/ethnicity, education, mar-
ital status, smoking, and health status [10-14]. In addi-
tion, subjects responded to a question regarding how long
it had been since their last doctor's visit for a routine
check-up.
Analysis
We used the two-sample t-test for independent samples to
evaluate differences in mean values of continuous varia-
bles (e.g., age and number of servings of fruits and vegeta-
bles per day) between the randomly-assigned study
groups. The chi-square test was used to analyze the cate-
gorical variables of interest. Multiple linear regression
analysis was used to assess the independence of the asso-
ciation between fruit and vegetable intake and the inter-
vention group, after adjusting for covariates (e.g.,
education, marital status). Multiple logistic regression was
used to assess the independence of associations between
intervention group and the categorical outcomes of
reported fruits and vegetables servings per day after adjust-
ing for potential confounding factors. SAS, Version 8.0
was used to conduct these analyses [15].
Results
Control and intervention subject groups were similar with
respect to most characteristics (Table 1). The majority of
participants in both groups were Non-Hispanic White and
married, and perceived their health as very good or good,
had received a medical check-up within the last year, and
were non-smokers. A slightly higher proportion of
women randomly assigned to the intervention group were
Non-Hispanic White (96% vs. 88%, P = 0.03) and college
graduates (64% vs. 44%, P = 0.05) than were women ran-
domly assigned to the control group.
Subjects assigned to the intervention group reported a sig-
nificantly higher mean intake of total fruits (including
juice) (2.0 vs. 1.3 servings per day, P < 0.001), total vege-Page 3 of 7
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jects responded with the number of times per day, week,
month, or year they consumed each food. From these
tables (3.2 vs. 2.4, P < 0.001), and total fruits and vegeta-
bles (5.2 vs. 3.7, P = < 0.001) than did the control subjects

Nutrition Journal 2008, 7:18 http://www.nutritionj.com/content/7/1/18
(Table 2). No difference was observed between the inter-
vention and control groups with regard to reports of
intake of milk, potatoes, or sweet products (doughnuts,
cookies, cake, pastries, or pies) (Table 2). A higher propor-
tion of intervention subjects were categorized as eating 5
or more servings of fruits and vegetables per day by the
FFQ (40% vs. 18%, P = 0.002 (Table 3). Adjustment by
multiple logistic regression for age, education level, race/
ethnicity, self-perceived health status, and time since last
check up had little effect on this association (Crude OR =
3.0, 95% CI 1.5 to 6.2; adjusted OR = 3.0, 95% CI 1.4 to
6.4) (Table 3). Multiple linear regression analyses also
showed no confounding by age, education level, race, self-
perceived health status, and time since last medical check-
up on servings per day (data not shown).
In responding to the 24-hour recall questions, a greater
proportion of intervention subjects reported consump-
Table 1: Summary of demographic and health characteristics of intervention and control groups
Control
Group
n = 83
Intervention
Group
n = 80
p-value
Age (years) 0.07
< 45 years 31 % 26 %
45–55 years 51 % 40 %
> 55 years 18 % 34 %
Race/ethnicity 0.03
Non-Hispanic White 96 % 88 %
Other 4 % 12 %
Education 0.05
Some HS or HS Grad 24 % 15 %
Some College 31 % 21 %
College Grad or Higher 44 % 64 %
Marital Status 0.14
Married 90 % 82 %
Not Married 10 % 18 %
Smoking Status 0.93
Smokers 12 % 12 %
Non-smokers 88 % 88 %
Self-Perceived 0.86
Health Status Excellent 26 % 26 %
Very Good/Good 65 % 68 %
Fair/Poor 8 % 6 %
Time Since Last 0.19
Medical Exam Within Past Year 84 % 76 %
More Than One Year Ago 16 % 24 %
Table 2: Reported mean intake of fruits, vegetables, and other foods collected by food frequency questionnaire
Mean Servings Per Day (± SD)
Control
Group
n = 83
Intervention
Group
n = 80
Difference P-value
Milk 0.70 (0.72) 0.88 (0.94) 0.18 0.18
Doughnuts, Cookies, Cake, Pastries, or Pies 0.32 (0.5) 0.35 (0.58) 0.03 0.66
100% Fruit or Vegetable Juice 0.43 (0.5) 0.64 (1.04) 0.21 0.10
Fruit 0.87 (0.69) 1.33 (0.95) 0.46 < 0.001
Carrots 0.30 (0.44) 0.38 (0.34) 0.08 0.21
Potatoes 0.24 (0.28) 0.23 (0.22) 0.01 0.97
Green Salad 0.66 (0.46) 0.83 (0.54) 0.17 0.03
Other Vegetables 1.2 (0.84) 1.8 (1.46) 0.6 0.001
Total Fruit and Juice 1.3 (0.89) 2.0 (1.47) 0.7 < 0.001
Total Vegetables 2.4 (1.26) 3.2 (1.86) 0.8 < 0.001Page 4 of 7
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Total Fruits and Vegetables 3.7 (1.66) 5.2 (2.67) 1.5 < 0.001

Nutrition Journal 2008, 7:18 http://www.nutritionj.com/content/7/1/18
tion of fruits and/or vegetables the preceding day at break-
fast (64% vs. 46%), lunch (74% vs. 59%), dinner (89% vs.
69%), and for snacks (41% vs. 25%) (Table 3). However,
only with regard to the dinner meal was the difference sta-
tistically significantly higher among the intervention sub-
jects. In total, however, a significantly greater proportion
of intervention subjects reported consumption of fruits
and/or vegetables on three or more occasions throughout
the previous day than did control subjects (61% vs. 32%,
adjusted OR = 2.8, 95% CI 1.6 to 5.2).
A higher proportion of subjects in the intervention group
considered their diets very high or high in fruits and vege-
tables than the control group (61% vs. 39%, P = 0.05).
Discussion
In this study, intervention subjects exposed to brief mes-
sages about the benefits of fruits and vegetables and 5-A-
Day guidelines for consumption in the day preceding die-
tary assessment reported substantially higher intakes of
fruits and vegetables by both the FFQ and the 24-hour
recall methods. Adjustment for factors known to be asso-
ciated with fruit and vegetable intake had little effect upon
the size of this difference. This difference in behavior
could reflect an actual (although most likely temporary)
change in behavior or an intention to change. However,
the minimal nature of the intervention and the large size
of the difference make this explanation unlikely. This
study therefore suggests that social approval bias might
well be a substantial problem in the interpretation of
magnitude of this bias is similar to the intervention effects
reported in many studies evaluating changes in fruit and
vegetable intake (ranging from 0.93 to 1.25 servings per
day) [16-18]. Thus, a major challenge facing nutritional
intervention researchers is assessing true behavioral
change based on self-reports from reporting bias.
This study employed brief dietary assessment methods
that are commonly used in nutritional epidemiologic
studies, including intervention trials [16-19]. The esti-
mates of mean fruit and vegetable intake per day among
control subjects from the food frequency portion of the
interview was 3.71 servings, which is consistent with base-
line or control mean intake amounts identified in other
studies as well as the National Cancer Institute 5-A-Day
program evaluation [19-21]. In addition, we observed an
association between fruit and vegetable intake and self-
perceived health status that is consistent with findings of
previous research [10].
We had hypothesized that the 24-hour recall might be less
susceptible to the effects of social approval bias because
answers are reported according to memory of the previous
day's intake instead of being derived from more generic
memory, as in the food frequency questionnaire [7].
However, we observed substantial biases in both types of
measures. More intervention subjects reported consump-
tion of fruits and vegetables than did controls for each
meal on the prior day, and substantially more therefore
reported eating fruits and vegetables on 3 or more occa-
Table 3: Reports of fruits and vegetables intake by food frequency and 24 hour recall methods.
Control
Group
(n = 83)
Intervention
Group
(n = 80)
Crude OR
(95% CI)
Adjusted OR
§
(95% CI)
FFQ report of five or more fruit & vegetable servings per day 18 % 40 % 3.0*
(1.5 to 6.2)
3.0
(1.4 to 6.4)
24 hour recall of fruits or vegetables intake the previous day at:
breakfast 46 % 64 % 2.0**
(1.1 to 3.7)
1.6
(0.85 to 3.1)
lunch 59 % 74 % 1.8**
(.96 to 3.5)
1.5
(.77 to 3.1)
dinner 69% 89% 3.6**
(1.2 to 7.8)
3.8
(1.6 to 9.4)
snacks 25% 41% 1.8**
(1.0 to 3.1)
1.8
(.91 to 3.1)
Sum of three or more times throughout the day 32% 61% 3.4**
(1.9 to 6.1)
2.8
(1.6 to 5.2)
*OR estimated via multiple logistic regression as the odds of 5 or more servings per day vs. fewer than 5 for intervention group as compared to
control group.
** OR is the odds of having reported eating a fruit or vegetable in the specific meal (yes vs. no), or 3 or more times in the day (vs. under 3 times),
compared to the control group, as estimated by logistic regression.
§
Adjusted by multiple logistic regression for age, education level, race/ethnicity, self-perceived health status, and time since last medical check-up.Page 5 of 7
(page number not for citation purposes)
nutritional intervention effects that are dependent on
education and awareness to affect behavior change. The
sions on the previous day (61% vs. 32%, p = 0.002). These
findings suggest that social approval bias may affect short-

Nutrition Journal 2008, 7:18 http://www.nutritionj.com/content/7/1/18
term dietary recall methods as well as more general
longer-term recall processes used for food frequency
reports.
Our findings are consistent with previously established
theories regarding the effects of social desirability and
social approval bias upon self-reports of usual food
intake. Prompting subjects in the intervention group
about the benefits of fruits and vegetables likely altered
their memory of food intake to be consistent with what is
considered "good for health". Responses about other food
items included in the food frequency questionnaire, such
as milk, potatoes, and sweet bread products, that may be
considered neutral or socially undesirable, were unaf-
fected by this apparent bias.
Results of this study were obtained from a small sample of
the Colorado female population. We were therefore una-
ble to assess the impact of reporting bias among men, or
among subgroups defined by age, race/ethnicity, or socio-
economic status. Future work should examine effects in
such subgroups, and should also assess the relationship
between the dose and timing of the biasing prompts, and
the resulting bias. More detailed assessments of full 24
hour diet recalls (rather than only the fruit and vegetable
specific limited recall used in this study) should also be
examined in future studies. Future work should also
examine the bias in reports of diet over time between
intervention and control subjects, where change is meas-
ured as the difference in the intervention-control differ-
ences over time. Any difference in fruits and vegetables
intake between baseline and follow up should be inter-
preted with caution as it could be due to either bias from
participating in the intervention, or to actual change.
Another explanation of the difference in reporting fruits
and vegetables intake reported in this study between the
study groups could be that the diets between these two
groups were really different, and not just on the day of the
interview. However, the randomized blinded nature of
this study makes this explanation unlikely. The difference
in the reported fruits and vegetables intake can also be
attributed to the intervention resulting in a temporary
behavior change in the intervention group. We believe it
is unlikely that such a small "intervention" could account
for such a large behavioural change.
Alternatives to self-reported measures of dietary intake in
the evaluation of dietary interventions deserve close atten-
tion and careful thought in the design of dietary interven-
tion studies. Methods of data collection that are more
objective in nature will reduce bias and provide more reli-
able estimates of intervention effects [22]. Alternatives for
throughout the intervention process, as used in the Polyp
Prevention Trial [23] and in an assessment of fruit and
vegetable intake in Norway [24]. Although use of nutri-
tional biomarkers may exhibit some limitations due to
cost and compliance, this more objective method of die-
tary assessment eliminates the concerns about bias associ-
ated with self-reported dietary intake [25,26].
In some settings, where intervention targets are specific to
confined groups of people, direct observation of dietary
intake before and after an intervention may be feasible as
an objective method of dietary assessment. For example,
direct observation of children eating in school settings, or
workers in worksite cafeterias can provide an objective
dietary measure for interventions expected to affect food
choices. For community-wide interventions, evaluation of
food purchases at markets or purchasing trends at dining
establishments are objective measures for interventions
expected to change purchasing patterns.
In large dietary intervention trials, comprehensive solu-
tions to eliminate the bias in self-reported dietary intake
may prove costly and unrealistic for all subjects. However,
subgroups can be assessed with biomarkers or other inde-
pendent assessments to at least estimate the size of the
reporting bias that might be expected to have been
induced by the intervention. Bias can also be controlled to
some degree by evaluating different intensities of an inter-
vention, in which everyone receives at least a minimal
prompt for change, rather than evaluating differences
between study groups exposed to an intense intervention
as compared to those not exposed to any intervention.
Conclusion
Our results show that self-reports of fruit and vegetable
intake by means of both food frequency questionnaires
and 24-hour recall are susceptible to substantial social
approval bias. Attention to this bias in the context of die-
tary reports from subjects in nutritional intervention stud-
ies is an important consideration in study design, analysis,
and interpretation. Continued efforts to improve meth-
ods to objectively evaluate nutritional interventions are
needed.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
TMM designed and coordinated the study, recruited sub-
jects, collected data, analyzed data, wrote the manuscript,
and helped edit the manuscript, MFA helped design the
study, provided significant professional consultation,
recruited subjects, collected data, and helped edit thePage 6 of 7
(page number not for citation purposes)
consideration may include use of nutritional biomarkers
such as carotenoids to assess changes in nutritional intake
manuscript, LAC helped design the study, edit the manu-
script, and provided consultation, ACM helped design the

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study, helped with the study coordination and editing the
manuscript, and provided study consultation, TEB helped
design the study, provided significant professional advice,
and helped write and edit the manuscript.
Acknowledgements
Support for this project provided by the University of Colorado at Denver
and Health Sciences Center.
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