This report has presented the first in a series of experiments investigating factors that influence or disturb the normal functioning of the maternal instinct in animal subjects. The present experiment was a more detailed study of a finding made previously: that maternal behavior in the rat varies inversely with the number of offspring in the litter.
Sixty female albino rats of the Wistar strain were used as experimental mothers. Their first litters were experimentally arranged so that one group of mothers had litters of 3 offspring, another group had litters of 6, another litters of 9, and a fourth group had litters of 12. Tests of maternal behavior toward the offspring revealed a stepwise, almost linear inverse correlation between litter size and maternal behavior. The second litters of the same mothers were arranged so that mothers with small first litters received large second litters, and vice versa. Tests revealed the expected increase in maternal behavior for mothers with smaller second litters. Mothers given larger second litters, however, failed to reveal an expected decrease in maternal behavior.
Hypotheses and alternative possibilities to explain these findings have been presented and discussed. Two hypotheses are presently favored by the author as explanations for the inverse relationship between litter size and maternal behavior. The pattern of high maternal behavior toward small litters and low maternal behavior toward large litters may be based upon an innate behavior-regulating mechanism ("instinct") which has been se-lected-in to the heredity of these animals because the pattern has survival value for the species. High maternal behavior toward small litters may promote survival by protecting against further loss of offspring in already small litters. Low maternal behavior toward large litters may promote survival by (1) protecting against excessive fatigue in the mother, upon whose health and strength the safety and survival of the entire litter depends, and (2) by promoting survival of only the fittest offspring in large litters. The second hypothesis was that fatigue induced in the mothers of large litters may serve as a physiologic signal mechanism, activating a stepwise reduction (inhibition?) of maternal behavior as the fatigue level rises.
To explain the absence of an expected decrease in maternal behavior toward larger second litters, two counteracting factors have been postulated: the factor of larger litter size, tending toward lower maternal behavior, may have been neutralized by the factor of maternal maturity and experience, which favor an elevation of maternal behavior with second litters.
The possible relation of these findings to human biology and behavior has been discussed briefly. The question arises whether the inverse relationship between maternal behavior and number of offspring may also tend to occur in human beings, and whether this may be a factor that tends in some instances to disturb the natural and optimal functioning of the maternal instinct in humans. It was suggested that some clinical and sociologic observations support the concept that there probably is a number of offspring in humans above which additional children may overburden the parent and produce a decrease in parental care.
Two implications of this line of reasoning for mental hygiene and preventive psychiatry have been suggested: the mental health of both parents and their children would be better served if (1) parents could be helped to develop more realistic attitudes about their capacities for parenthood, and if (2) parents could be given more help medically, psychiatrically, legally, and morally in overcoming the feeling of necessity to have children that they do not want. The present studies of the maternal instinct in animal subjects suggest that there may possibly be an innate biological basis and mechanism for the tendency toward reduction of parental care in reaction to large numbers of offspring.