1. Unbounded rationality assumes that there are no limits for information search, while the three other approaches assume that search must be limited because real decision-makers have only a finite amount of time, knowledge, attention, or money to spend on a particular decision.
2. Optimization under constraints assumes that the mind should calculate the costs and benefits of searching for each further piece of information and stop as soon as the costs outweigh the benefits. Gigerenzer, Todd et al. regard this vision of rationality as something that comes very close to unbounded rationality:
The paradoxical approach of 'optimization under constraints' is to model 'limited' search by assuming that the mind has essentially unlimited time and knowledge with which to evaluate the costs and benefits of further information search. ... constrained optimization invites unbounded rationality to sneak in through the back door. (Gigerenzer, Todd et al. 1999:11)
3. Satisficing is a method for making a choice from a set of alternatives which takes the shortcut of setting an adjustable aspiration level and ending the search for alternatives as soon as is encountered that exceeds the aspiration level (Gigerenzer, Todd et al. 1999:13),
4. Fast and fruga heuristics 'make their choices with easily computable rules' (Gigerenzer, Todd et al. 1999:14). 'A computational model of an heuristic specifies the precise steps of information gathering and processing that are involved in generating a decision such that the heuristic can be instantiated as a computer program' (p. 16)." - Bofinger, p. 243-4
"The difference between Svensson's approach and the approach that is used in the following is neatly described by Gigerenzer, Todd et al. (2000): 'Leibniz' universal calculus exhibits the aesthetics and the moral virtue of this lofty ideal, as does Laplace's omniscient superintelligence. Cognitive scientists, economists, and biologists have often chased after the same beautiful dreams by building elaborate models endowing organisms with unlimited abilities to know, memorize, and compute. These heavenly dreams, however, tend to evaporate when they encounter the physical and psychological realities of the waking world: mere mortal humans cannot hope to live up to these standards, and instead appear nightmarishly irrational and dysfunctional in comparison. In the face of this dilemma, many researchers have still preferred to keep dreaming that humans can approximate the exacting standards of optimality, rather than surrendering to an ungodly picture of human irrationality and stupidity. The choice, however, is not between an unrealistic dreaming rationality and a realistic nightmare irrationality. There is a third vision that dispenses with this opposition: rationality through simplicity, and accuracy through frugality.'
- Bofinger, p. 245, footnote