In theory at least, the exchange is non-forcible, but the reality of the effects of these exchanges has always been unequal, and at least one half of the so-called "exchange" has usually been forced by the stronger or richer participant.
En teoría polo menos, o intercambio non é forzoso, pero a realidade dos efectos destes intercambios foi sempre desigual, e polo menos a metade do chamado "intercambio" foi forzada polo participante máis forte ou máis rico.