Contents - Konrad Lorenz Institute

A Test for Comprehension of False Belief in Chimpanzees
Correct Responses (%)
Correct Responses (%)
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current
previous
other
3-yr 4-yr 5-yr 6-yr Autistics Chimps
3-yr 4-yr 5-yr 6-yr Autistics Chimps
Figure 4. Percentage of choices made for the previous and current
peg positions and the other two positions (combined) in
the mechanical analogue task. Top: True belief version. Bottom:
false belief version.
SMIRNOV tests, P = 0.104 and P = 0.083 respectively;
however, as a check, we ran a non-parametric
ANOVA, but the results were the same: P = 0.042
and P = 0.003 respectively.) Bonferroni post hoc tests
confirm that six-year-olds score significantly higher
(P ≤ 0.027 2-tailed) than all other groups of subjects
except the chimpanzees (for whom P = 0.48 2-
tailed) on the false belief tasks, but not on the true
belief tasks (except against the chimpanzees, where
P = 0.058 2-tailed). Conversely, the chimpanzees
differ significantly (P ≤ 0.058 2-tailed) from all the
human subjects (except the five-year-olds and autistic
subjects, for whom P ≥ 0.38) on truth belief tasks,
but not on the false belief tasks.
An alternative way of considering the results is to
compare the number of occasions when subjects selected
the previous and current positions of the pegs
in the true belief and false belief trials. Subjects
should choose the current position in true belief trials
and the previous position in false belief trials.
Figure 4 plots the mean percentage of choices for
the four drawers (summing the scores for the two
“other” choices). It is clear from these results that
only the 6-year-olds showed the correct switch from
current to previous peg position between the two
types of trials. Younger children and the autistic
adults all exhibited a strong tendency to select the
previous position of the peg in both types of trial.
Significantly, perhaps, chimpanzees exhibited a
weak tendency to switch choices correctly with trial
type, even though they made many more incorrect
decisions (“other” drawers) than any of the human
subjects. However, only one chimpanzee (Josie) selected
the previous peg position on false belief trials
and the current one on true belief trials significantly
more often than expected by chance (“other”
choices discounted: 2 × 2 χ 2 = 21.88, df = 1,
P < 0.001). Matched pairs t-tests confirmed that, as a
group, only the 6-year-olds chose the previous peg
position significantly more often in the false belief
trials (t = 2.42, df = 9, p < 0.05) and the current peg
position significantly more often in the true belief
trials (t = 2.87, df = 9, p < 0.02); all other comparisons
were non-significant. This suggests that, even
though their performance was far from perfect, the
chimpanzees had a better grasp of the false belief
task than either the autistic adults or the younger
children.
Learning and task performance in chimpanzees
The chimpanzees were given extended trials on
both the true belief and false belief tests in order to
ascertain the extent to which learning might play a
role in their responses to the tasks. Regression analyses
of the results using successive blocks of 10 trials
on each test type are summarised in Table 3.
Pooling results using Fisher’s procedure (with 1-
tailed P-values in a positive direction to test for an
underlying trend for scores to improve with time)
yields χ 2 = 4.56 (df = 2 × 4 = 8, P = 0.80) for the true
belief task and χ 2 = 7.35 (df = 8, P = 0.50) for the
false belief task, indicating that there is no underlying
trend in either case. Indeed, the mean slope on
Subject slope r2 t df* P
(a) True belief trials:
Pepe –0.004 0.00 –0.035 11 0.959
Josie 0.600 0.14 0.707 4 0.530
Flynn –0.100 0.01 –0.141 4 0.897
Beckie 0.114 0.01 0.231 4 0.829
(b) False belief trials:
Pepe –0.025 0.01 –0.273 11 0.717
Josie 0.600 0.53 0.187 4 0.164
Flynn –0.800 0.46 –1.589 4 0.210
Beckie –0.400 0.38 –1.572 4 0.191
Table 3. Regression statistics for a linear trend in correct scores
with blocks of trials for chimpanzees to test for a learning effect.
*blocks of 10 test trials.
Evolution and Cognition ❘ 137 ❘ 2003, Vol. 9, No. 2