This document summarizes a study that used an iterated learning model to explore how cultural evolution occurs. The study found that merely observing the behavior of others is not enough for cumulative cultural evolution, and that people must communicate their theories and beliefs. This suggests language allows humans to uniquely engage in cultural ratcheting by transmitting learned representations across generations.

1. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Iterated Learning and the Cultural Ratchet
Aaron Beppu Tom Griffiths
Department of Psychology
University of California Berkeley
Cognitive Science, 2009
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

2. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Outline
1 The Cultural Ratchet
2 Iterated Learing and variations
3 Results
4 What does this mean?
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

3. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
What is the Cultural Ratchet?
The process of cumulative cultural evolution requires
not only creative invention but also . . . faithful social
transmission that can work as a ratchet to prevent
slippage backward – so that the newly invented artifact
or practice preserves its new and improved form at
least somewhat faithfully until a further modification or
improvement comes along.
– Michael Tomasello, The Cultural Origins of Human Cognition
we do this
other animals don’t
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

4. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
What are some existing explanations?
We can try to explain this by focusing on individual teachers
and learners :
We’re sophisticated learners (e.g. Tomasello 2001)
We learn strategies and goals from each other
We can follow each others attention
We can faithfully reproduce behaviors we learn from others
We’re also good teachers (e.g. Gergeley & Csibra 2005)
We understand which things are hard to learn
Teachers call learner’s attention to the most relevant parts
Human learners benefit from an enriched learning
environment
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

5. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Modeling Cultural Evolution
hypothesis hypothesis
data data data ...
Learners are ordered into a sequence, and each agent learns
from the output of the previous agent
Can be used to study change in culture, (e.g. evolution of
language Kirby 2001), or to examine people’s biases (e.g.
Griffiths, Christian & Kalish, 2006)
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

6. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Analyzing Iterated Learning
Specifics:
Agents are Bayesian, and all have the same prior. Bayes’ Rule :
p(h)p(d|h)
p(h|d) =
h p(h )p(d|h )
The data that a given agent passes on is sample according to
his posterior beliefs. p(di |di−1 ) = h p(di |h)p(h|di−1 )
The pure iterated learning model converges to the prior beliefs
of its agents.
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

7. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Cumulative Cultural Evolution
Cumulative cultural evolution will require constantly bringing in
new data from the world, in addition to some information
passed from one generation to the next. We vary what kind of
information is passed between agents.
hypothesis hypothesis
??? ??? ??? ...
new data new data
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

8. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Observational Learning
hypothesis hypothesis
... data data data ...
new data new data
In the “Mixed Data” case, each agent receives new data from
the world, as well as data points sampled from the posterior
distribution of the previous agent. This is similar to
observational learning, achieved by observing the behavior of
another.
In this case, convergence is to d ∗ p(h|d ∗ )p∗ (d ∗ ).
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

9. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Communicating Theories
hypothesis hypothesis
theory theory theory ...
new data new data
In the “Posterior Passing” case, each agent receives new data
from the world. In addition, rather than receiving samples from
the previous person’s posterior distribution, each agent now
receives the whole posterior distribution. Communicating
theories might approximate how humans teach/learn.
This chain converges to the truth.
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

10. The Cultural Ratchet
Iterated Learing and variations
Iterated Learning
Results
What does this mean?
Function learning is hard
Why function learning?
Strong bias towards positive linear functions; non-monotonic
functions are hard (Busemeyer, Byun, DeLosh & McDaniel,
1997)
The pure iterated learning case converges to a linear function
(Kalish, Griffiths, Lewandowsky, 2007)
We can select a function that is too difficult for a single person
to learn on their own, but which could plausibly be learned via
cumulative cultural evolution.
By using a difficult to learn parabola, we can separate out
groups of people converging to the posterior from people
converging to the prior.
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

11. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Apparatus
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

12. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Examples from our results
Training Testing
Pure iterated learning
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

13. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Training Testing
Pure iterated learning
Mixed data
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

14. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
Training Testing
Pure iterated learning
Mixed data
Posterior passing
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

15. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
0.6 pure iterated learning 0.6
mixed data
posterior passing
0.5 0.5
0.4 0.4
RMSE
RMSE
0.3 0.3
0.2 0.2
0.1 0.1
0 0
0 2 4 6 8 0 2 4 6 8
Generation Generation
Left : Cumulative average error (RMSE) for each chain for each
generation. Right : Same as left, but averaged by condition.
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet

16. The Cultural Ratchet
Iterated Learing and variations
Results
What does this mean?
What do we walk away with?
Merely observing behavior of those around us isn’t enough to
have cumulative cultural evolution
We need to communicate theories and beliefs about the world.
Language is one way we can do this.
This gives us an idea about the uniquely human characteristics
that facilitate cultural evolution.
What next?
Can we adequately recover the posterior distributions from our
teachers merely from observing their actions?
What can we do with richer teacher/learner interactions?
How can we reconcile this inference based perspective with
other pressures on cultural evolution?
Aaron Beppu, Tom Griffiths Iterated Learning and the Cultural Ratchet