Ball J.A. The Zoo Hypothesis, dans Icarus, vol. 19, no 3, juillet 1973, p. 347—349. — ISSN 0019-1035 et 1090—2643
Extraterrestrial intelligent life may be almost ubiquitous. The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.