There are no genes that by themselves directly create instincts or other anlagen of behavioral patterns destined to unfold as dimorphically male or female. Likewise, there is no prenatal hormone – nor any other known substance or combination of substances – that directly creates sexually segregated and dimorphic instincts or forms of reasoning or other behavior. It does appear, however, on the basis of evidence so far available, that prenatal hormones (which themselves may be genetically regulated) are able to establish trends or dispositions toward a few sexually dimorphic forms of behavior. The principle is that a given hormone changes the threshold for the eventual emergence of a certain type of behavior. Threshold is the key word (Chapter 3). It is not that males are endowed with one kind of behavior and females with another. The behavior is shared. It is the threshold for its emergence that is sexually dimorphic....

The issue is whether the sex-different threshold is a product simply of postnatal conditioning and habituation, or whether it is set by a prenatal, species-shared phyletic mechanism. As in many other fundamentals of human behavior, the issue cannot yet be resolved conclusively, for systematic research is exceedingly sparse. However, there is an important lead to be found in what was originally a serendipitous experimental finding of Koranyi and his associates in Pécs, Hungary.

The experiment as planned was designed to monitor under various conditions electrical activity of multiple cell units (multiple unit activity) in different brain regions, including the anterior hypothalamus. The experimental animals, cats, lived with microelectrodes permanently and safely implanted into the brain. They were tested on many different occasions. It so happened that one experimental animal became pregnant and delivered a litter of kittens. Experimentally, she was too valuable to exclude from further study. So, she was brought with her kittens into the testing room. There, on one occasion, it was observed that the recording traced on paper by an EEG (brainwave) pen was highly erratic. This erraticism could have been written off as an experimental error. Instead, the investigator was astute enough to recognize that it occurred only when the hungry kittens in the litter box meowed, calling for their mother to feed them. The sound of their hungry call triggered a response in various parts of the brain, particularly, as subsequent investigation showed, in the amygdala. Subsequent investigation showed also that a mother cat’s sensitivity to the call of her kittens rose sharply during the period of lactation. This increase in sensitivity is regulated by lactogenic hormone or possibly some other substance in the bloodstream. This point was proved using mice as the experimental animals.

When plasma from a lactating mouse-mother was injected into virgin females or into males, it produced a dramatic change in their parental behavior: primed with lactogenic plasma, these other animals required only twelve hours, instead of seven to nine days, of exposure to helpless young pups in order to become maternally responsive to their distress signals. In other words, the lactogenic plasma effect worked by lowering the threshold of stimulus sensitivity and response.