the opposite. Acting as a centripetal force, inferences and memories
lead to roughly similar constructions even though the input may be
quite different. This is why we can observe similarities between con-
cepts both within a group—my notions about animals are quite similar
to those of my relatives—and also between groups—there are impor-
tant similarities in animal concepts from Congo to Greenland,
because of a similar template.
At about the same time as meme-accounts were devised to
describe cultural transmission, Dan Sperber and some colleagues put
together anepidemiologicalframework to describe the mechanisms of
[46] cultural transmission. The substance of this framework is what I just
explained in terms of information and inference. An epidemic occurs
when a group of individuals display similar symptoms—when for
instance people in a whole region of Africa get high fevers. This is
explained as an epidemic of malaria, caused by the presence of mos-
quitoes carrying the Plasmodium pathogen. But note that what we
call the epidemic is the occurrence of fevers and assorted symptoms,
not the presence of mosquitoes or even Plasmodium. That is, to
explain what happened you must understand the particular ways in
which the human body reacts to the presence of this particular
agent. If you do not know any physiology, you will have a hard time
explaining why only some animals catch malaria, why fewer people
with adequate preventive treatment catch it than do others, or
indeed how the disease spreads at all. We may well study the struc-
ture of Plasmodiumforever; this will tell us nothing about its effects
unless we also learn a lot about human physiology. Mental represen-
tations are the effects of external vectors, mostly of communications
with other people. But then the structure of the messages exchanged
does not by itself tell us how the mind will react to them. To under-
stand that, we must know a lot about human psychology, about the
way minds produce inferences that modify and complete the infor-
mation exchanged.^12
TOOL KIT 2: CULTURAL EPIDEMICS
Human minds are inhabited by a large population of mental
representations. Most representations are found only in one
individual but some are present in roughly similar forms inRELIGION EXPLAINED