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171 Those implacable replicators will either have or lack a built-in prin- ciple of reproductive restraint, proposed Sagan and Newman. If their reproduction is limited, they would not be everywhere. If it were not limited, they would threaten the culture that contemplated making them and would not have been made. We may be seeing forerunners of this issue in the development of robot fighting machines for Earthly combat. “The lawyers tell me,” said one military researcher, “there are no prohibitions against robots making life-or-death decisions.”” Mere possession of the technology for expansion is not enough. The moti- vation to expand must also be there. - 190930 Models of expansion like Tipler’s rest on extreme assumptions about what societies of intelligent beings would do, taking for granted that expan- sion is both inevitable and continuous. One of the arguments used by expansion theorists already has been undermined by events in the real world. As a driving force, population growth is far less credible today than it was 30 years ago; we know now that the population explosion can be slowed, even halted. Finney, comparing interstellar colonization with historical human migra- tions (particularly the Polynesian expansion across the Pacific Ocean), concluded that “no specific migration has ever gone unchecked. Ecological barriers, the slowing or cessation of innovation, flagging motivation, or the opposition of those in the way have...stopped every... colonization movement so far.” Interstellar colonization would not fill the entire Galaxy.*! Judging by human history, expansion episodes may be brief. Chinese explorers made many long voyages in great ships while the European Age of Exploration was just getting under way. Evidence suggests that they reached Africa; according to one controversial theory, they landed in the Americas well before Columbus and may even have planted colonies. Despite these magnificent achievements, Chinese officials made a policy decision to stop the voyages. The hulks of China’s exploring vessels were left to rot away on riverbanks. The colonies—if there were any—disappeared. Only a small minority of our own species favors extending permanent human presence beyond the Earth. Extraterrestrials also might have a wide variety of drives, abilities, and situations. Expansion, if begun, might be episodic and may be sustained for only for limited periods of time; it might cease for societal reasons. The Psychology of Expansion The Psychology of Expansion —Ben Finney, 1983”