The nesting instinct

According to PregnancyWeekly, around the fifth month of pregnancy, a woman develops a set of nesting behaviors (cleaning house, etc) that on the surface seem quite rational (preparing for the new baby, etc).

But in reality, the behaviors are a "primal instinct" (like "birds making their nests"), not a rational choice based on a logical deduction (i.e. "I need to clean up to ensure the health and safety of the baby").  We know this, because the woman's behaviors themselves are not rational, and are often counterproductive:

Women have reported throwing away perfectly good sheets and towels because they felt the strong need to have "brand new, clean" sheets and towels in their home. They have also reported doing things like taking apart the knobs on kitchen cupboards, just so they could disinfect the screws attached to the knobs. Women have discussed taking on cleaning their entire house, armed with a toothbrush ... Being preoccupied with ant killing, squishing them one at a time for weeks on end. Packing and unpacking the labor bag 50 times. Cleaning the kitchen cupboards and organizing everything by size to the point that you make sure the silverware patterns match when it's stacked in the cutlery drawer. Sorting the baby's clothes over and over again is a favorite theme. Taking them out of the drawers and re-folding them, putting them away and doing it over and over again.

Clearly, many of these behaviors are obsessive-compulsive, and may actually jeopardize the baby's health by putting stress on the mother.  Because it's not rational, it must be instinctive.  But how do instincts work?  Some of our genes must develop the circuitry in a woman's brain to make the instinct function properly:

1. Genes develop the brain circuitry to motivate women to compusively "clean up" and organize
2. Other genes create an "on switch" that allow simple pregnancy hormones to activate the behavior at the appropriate time

A Nobel prize awaits the scientist who can explain how instincts are manifest in human genes and neurons in the brain.

Comments

[Craig]

1. Instincts may be quite rational.
2. Genes may not be responsible for "circuitry" but a more accurate explanation would be genes being responsible for hormones and the hormonal stages of reproduction evolved over time that result in behaviors that in the average expression increase the likelyhood of survival of offspring. Nesting definitely fits this category.
3. These nesting behaviors are common in other mammals that reproduce quite rapidly and have more than one offspring and thus better lend themselves to genetic study. IE: known inheritance and mendellian ratios.
4. Mice and rabbits are perfect examples of these traits and some of the known problems/extremes with these nesting behaviors. It is well known amongst rabbit and mice breeders that the daughters of mice and rabbit mothers that express these problems produce daughters with the same behaviors/problems.
5. Some of these skills may also be environmental/learned. Even the daughters of good mothers may have difficulty if something happens, like a C-section. A good example is the mothering skills of female dogs that are born C-section to otherwise sound mothers.
These new mothers tend to be more detached and lack some of the skill of their mothers as opposed to new mothers from the same dam from normal vaginal births because C-section mothers sometimes have difficulty mothering after surgury.
This is clearly learned/environmental as the genotype of mother and female pups (on average) are largely the same.