It was in this stressful environment that Jennifer spent her infancy. Jennifer became suspicious of everyone and by age six started telling her older sister that their kind and beloved new stepfather was certainly cheating on their mother. Jennifer was sure of it and repeated her suspicions frequently. Lisa, finally went to their mom and asked if it were true. Their new stepfather was one of those men who just didn't have it in him to cheat, and Sheila knew it. She couldn't figure out why her younger daughter had become so anxiously fixated on the imagined infidelity of her new husband. But Jennifer's nervous system had imprinted the unsafe perceptual reality of her earliest years, so even good people seemed unreliable and threatening. The two sisters were raised by the same mother but under different circumstances, so one daughter's brain circuits had incorporated a nurturing, safe mom and the other's a fearful, anxious one.
The "nervous system environment" a girl absorbs during her first two years becomes a view of reality that will affect her for the rest of her life. Studies in mammals now show that this early stress versus calm incorporation - called epigenetic imprinting - can be passed down through several generations. Research in mammals by Michael Meaney's group has shown that female offspring are highly affected by how calm and nurturing their mothers are. This relation has also been shown in human females and nonhuman primates. Stressed mothers naturally become less nurturing, and their baby girls incorporate stressed nervous systems that change the girls' perception of reality. This isn't about what's learned cognitively - it's about what is absorbed by the cellular microcircuitry at the neurological level. This may explain why some sisters can have amazingly different outlooks. It appears that boys may not incorporate so much of their mothers' nervous system.
Neurological incorporation begins during pregnancy. Maternal stress during pregnancy has effects on the emotional and stress hormone reactions, particularly in female offspring. These effects were measured in goat kids. The stressed female kids ended up startling more easily and being less calm and more anxious than the male kids after birth. Furthermore, female kids who were stressed in utero showed a great deal more emotional distress than female kids who weren't. So if you're a girl about to enter the womb, plan to be born to an unstressed mom who has a calm, loving partner and family to support her. And if you are a mom-to-be carrying a female fetus, take it easy so that your daughter will be able to relax.
So why is a girl born with such a highly tuned machine for reading faces, hearing emotional tones in voices, and responding to unspoken cues in others? Think about it. A machine like that is built for connection. That's the main job of the girl brain, and that's what it drives a female to do from birth. This is the result of millennia of genetic and evolutionary hardwiring that once had - and probably still has - real consequences for survival. If you can read faces and voices, you can tell what an infant needs. You can predict what a bigger, more aggressive male is going to do. And since you're smaller, you probably need to band with other females to fend off attacks from a ticked off caveman - or cavemen.