1. Introduction

Human reproductive behavior is studied by specialists of many disciplines. Until now, however, there has been no convincing explanation of the fall in birth-rate in developed countries. Demographic investigations show that many modern couples are limiting themselves to one child, even as men and women express in opinion surveys the desire for larger families. This discrepancy has been explained by the fact that more than 50% of respondents said they would prefer a family with two children, one boy and one girl. That is to say that spouses would be more willing to have a second child, on condition that they could be assured of having either a son or a daughter. The impossibility of determining the sex of a future baby has thus proved a negative influence, decreasing the likelihood that couples will decide to have a second child.

preferable family structureused questionnaire

The possibility of regulating the gender of a child before conception has been of interest to human- kind since the dawn of time. We know of many recommendations, drawn from the sexual cultures of many nations, for how a woman might choose the sex of her baby. Of course, many of these recommendations should be met with skepticism - they are based more on mysticism than on scientific knowledge. The fact remains, however, that they are taken seriously by many people even today.

Unfortunately, the current theme has not been the subject of broad scientific investigation, and any progress towards its resolution has invariably led to heated discussions, reflecting the polarity of opinions on the only 100%-effective means of controlling the birth-rate. Some people have worried that artificial regulation of a child's gender may lead to social and demographic problems. Some authors felt that the disruption of the more-or-less stable proportion of male to female births could harmfully affect the rate of reproduction of the population (4.c. 140), or even lead to social up-heaval (5). The counter to this view is the confirmation (on the basis of sociological studies of reproductive attitudes) that an effective method of gender-determination will not lead to a disruption in the secondary sex ratio (6, c. 75). Perhaps this is really the case, because the majority of respondents express a preference for families with one boy and one girl. So, if we provide parents with the opportunity to realize their reproductive rights, there will not be any great change in the ratio of the sexes within the population. The population itself, however, may well be affected, increasing in countries where couples had previously limited themselves to one child, and decreasing in those where people usually have more than two.

Few specialists doubt that the gender of a child is at least partly determined by non-random factors (in fairness, it must be noted that some have disputed this in doctoral dissertations - 7). Investigations in various branches of science have provided data on the planning of a child's gender, and methods for gender planning have been proposed (8, 9, 10). None of these theories or methods, however, gives a convincing explanation of why the process of pre-zygotic determination should exist in nature.

The data show that this process has an effect on the population, and that it is a natural consequence of the division of a species into two genders. No single scientific discipline can provide sufficiently exhaustive evidence to convincingly demonstrate the biological advantage of this phenomenon. As a result, a multi-disciplinary investigation is necessary. First, we will clarify the influence of immunological factors.