Representatives of both economic and biological sciences have for a long time been struggling with a problem posed by empirically observable manifestations of altruism as a potentially irrational form of behaviour leading to a cut in individual consumption levels and being contrary to natural selection principles. Many economic models are based on the concept of material egoism of individuals, assuming that they maximise their individual consumption levels irrespective of the impact exerted by their behaviour on others’ consumption. This article attempts to present altruistic behaviour within the framework of one modern economic model and, by means of an interdisciplinary approach and on grounds of biological sciences, to prove rationality of that behaviour towards relatives by a person acting altruistically. To this end, William Hamilton’s biological theory of kin selection is presented in the first place. It is followed by Gary Becker’s model of economic altruism, proving that altruistic behaviour may lead to an increased level of an altruist’s consumption and translating the economic utility function into biological adaptation categories. Within the above-mentioned model it is attempted to prove that even individuals whose utility function does not involve interdependence with consumption of other individuals may take altruistic behaviour within the framework of manifestations of individual egoism. This is the case because they anticipate a positive secondary impact of altruistic behaviour on their own consumption. In another step, by means of enlargement of the Becker model into implication of the Hamilton law, the author proved rationality of an increased scope for altruism than is the case with the original version of this model, due to taking into account genetic interests leading to reproduction of someone’s genes thanks to relatives supported by that person.