In research practice, it is possible to observe natural selection at work by analysing fertility and mortality. Crow’s index takes into account both of these vital statistics components and allows a quantitative estimation of the operation of natural selection on the basis of demographic birth and death figures. In this study, we use the classical Crow’s index to determine whether the disintegration of the Slovincian population in the second half of the 20th century was caused by factors of a biological nature, finally leading to disturbances in the reproductive strategy, or whether it was a result of the impact of many factors of a cultural nature. Use was made of measuring cards for 109 women and 38 men. The sample was divided into two generations: 1st generation, or individuals born up to the year 1900, and 2nd generation—those born after1900. Inthis material the opportunity for the operation of natural selection due to differential mortality and differential fertility was rather weak. Both generations of Slovincians were characterized by high fertility, suggesting their non-Malthusian type of reproductive strategy, and decreasing mortality of sexually immatures over time.It seems, therefore, that the loss of ethnic identity by Slovincians and their migration from the home territories was influenced by several factors of a cultural nature rather than their biological distinctness.