In order to evaluate the effect of social and biological factors on family size (FS) and secondary sex ratio (SSR) in the families that they would desire to procreate as well as in the ones to which they belong, 1,518 unmarried persons of both sexes, students and workers, who reside in the metropolitan area of Monterrey, Mexico were interviewed. It was found that the average FS desired by the unmarried males and females, independently of their socioeconomic level and years of schooling, were

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... 9 and 3.04, which are lower than half the FS of 6.81 observed in the families to which they belong. Both males and females in all the socioeconomic levels and years of schooling, would desire to procreate a larger number of male children: SSR of 0.576 and 0.524, which are higher than the 0.498 observed in the families to which they belong. Also they manifested a strong preference for the birth order of the sex of their children, being of males for the first (SSR of 0.924 and 0.838) and of females for the second (SSR of 0.301 and 0.269). The fact that all children would be of the same sex did not change the desired FS in about 80% of the persons, but in the remainder 20% the desired FS would be increased to an average of five children. In order to reduce the growth rate it would be convenient to offer special counseling on family planning to these persons. In conclusion, the obtained information indicates that aside from the socioeconomic, cultural and biological factors, probably the family planning programs have had a positive influence on the persons that in the near future will start their reproductive stage, and it is possible to anticipate a reduction on the growth rate for the next generation.