Los efectos del Derecho supletorio son extremadamente importantes. El número de pacientes mantenidos artificialmente vivos es mucho más elevado si uno tiene que declarar expresamente que no desea que lo mantengan vivo conectado a una máquina (de manera que, si no lo hace, lo mantienen conectado) que, salvo que así lo exprese, no se le prolongue la vida artificialmente. El número de pactos de no competencia en compraventa de empresas será muy inferior si se considera implícito por efecto del art. 1258 CC que si se entiende lo contrario. Cuáles sean los efectos de la regla supletoria depende de muchas circunstancias pero, básicamente, lo decisivo es si el coste para los individuos de derogar la regla dispositiva es elevado o no. Una primera consecuencia de este razonamiento es que, como regla general, la norma supletoria/dispositiva debería responder a lo que la mayoría de los individuos decidirían o pactarían si pensasen - negociasen sobre la cuestión (los costes totales de derogar la norma se minimizan). Por ejemplo, en relación con la donación de órganos para transplantes. Léase el siguiente párrafo
"In the U.S., the waiting list for organs has grown from 13,000 in 1987 to 44,000 in 1996; meanwhile, the number of potential donors has decreased. Over one-third of those on the waiting list will die before an organ is found. Why do so few people bother to opt into organ donation programs? Is it simply that they have a preference not to donate? Few people see it as immoral if they choose not to donate their organs upon death or release the organs of deceased family members. Since people are allowed to make their own choices regarding their body, perhaps people are making the right decision, consistent with their preferences. According to Baron (1998), however, the issue is more complicated than that. Baron (1998) points out that the omission bias leads people to believe that they are being completely moral even if they pursue their narrow self-interest, as long as they obey a list of prohibitions. This explains the interesting finding that people tend to feel bad if they do something that causes harm, but do not feel so bad if they fail to do something that would have kept harm from occurring. Clearly, this suggests that individual preferences are not entirely consistent. Furthermore, this suggests that a change in how our organ donation system operates could save countless lives while still allowing individuals to make unconstrained choices that are consistent with their view of their own preferences. In the current organ donation system, the default is that a person’s organs will not be donated. Those who wish to donate need to “opt-in” to the donation system. If the United States switched to a system in which all eligible Americans donated their organs by default unless they noted otherwise (“opt-out”), we could reduce unnecessary deaths due to organ shortages virtually overnight (Bazerman, Baron and Shonk 2001; Johnson and Goldstein 2003). Indeed, this simple policy change could save the lives of 6,000 U.S. citizens every year, twice the number who died as a result of the attacks on 9/11. The evidence for this is striking: Johnson and Goldstein (2003) document that European countries with an opt-in program similar to that of the United States have donation rates that fall between 4% and 28%. In contrast, European countries with opt-out programs have rates ranging from 86% to 100%. Even if changing the U.S. organ donation system resulted in an increase only half the size of what is suggested by the European experience, no organ shortage in the United States would exist".