If you want to pass on your genes, you might hold out for a quality mate and raise your offspring with care. On the other hand, you might just enjoy promiscuous sex and let the pups fend for themselves.

What determines which strategy you’ll choose? Your mother — or rather, the early life experience she provides. That, and how your genes fold. At least it’s that way for rodents.

At the annual meeting of the American Psychiatric Association, Michael Meany, a psychobiologist at McGill, presented the results of years of work on mothering, genes, and behavior in rats.

If a rat mother is nurturant (she tends to lick and groom her pups, making her “high LG”), her offspring will be less anxious, better at facing stress, and good at parenting their own broods. Low LG mothers produce does who encounter puberty early, show greater sexual receptivity, enjoy an increased pregnancy rate, and neglect their young. Subsequent generations of female offspring will do the same — unless a lucky pup is “cross-nurtured” by a mother who licks and grooms, in which case the cosseted rat will grow up to be be calm and picky.

These responses typify two Darwinian strategies. If the early message is that life is nasty, you pass your genes on any which way. If you begin with a quality environment, you adopt a more patient approach.

The personality variants result from the way the brain handles the usual suspects: stress-responsive hormones, serotonin, and oxytocin. But in Meany’s rat model, at the base of the difference is an extremely simple difference in epigenetics.

Remember epigenetics? It refers to changes in the configuration of chromosomes that maintain the same gene sequences. Here, neglectful rearing results in the methylation of single base, a cytosine, found in a “non-coding” region of DNA that turns out to influence the production of receptors for stress hormones, via a “glucocorticoid receptor promoter.” It's not your genes, it's how they're folded, a geometry that encodes the trouble you've seen.