That's why cables of passenger elevators 

 have higher safety factors than those of 

 dumbwaiters: the liability judgment 

 against the elevator company will be much 

 higher if a snapped cable kills hotel guests 

 than if it just drops their room-service 

 breakfasts. Structures made of wood have 

 to have higher safety factors than those 

 made of steel because wood's strength is 

 initially more variable and deteriorates 

 faster with time. Safety factors are set high 

 enough to minimize the risk of structural 

 failure, but low enough to avoid unneces- 

 sary expense or size. That is, safety factors 

 reflect an optimization decision, based on 

 trade-offs between costs and benefits. 



Engineers used to make those decisions 

 empirically and often unconsciously. For 

 example, in New Guinea's Star Moun- 

 tains, from which I remmed a few months 

 ago, people still cross mountain torrents 

 over bridges that they build out of hanas 

 and saplings. Falling into the torrent 

 would mean certain death, so they build 



their bridges strong enough to make col- 

 lapse unlikely, but they also don't make 

 unnecessary work for themselves with 

 overbuilt bridges. Those principles of 

 bridge design evolved by natural selec- 

 tion, through experience with bridges that 

 did or did not collapse. 



In industrial societies, safety factors are 

 instead calculated from physical princi- 

 ples and are written by law into design 

 codes. However, those conscious deci- 

 sions are still ultimately framed by a 

 process of natural selection, where the 

 arena is the marketplace of competing 

 manufacturers and the selective agent is 

 consumer choice. Builders presumably 

 cease to buy elevators from companies 

 whose cheap cables snap. They also cease 

 to buy from companies whose overde- 

 signed elevators cost double the price of 

 already-safe, competing elevators. 



Biological safety factors similarly 

 evolved through natural selection, but the 

 process is always "unconscious," and the 



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