Mother’s Day is supposed to give moms a break from their regular routine of feeding kids, cleaning up spills, helping with homework, chauffeuring to activities, and-for a majority of mothers-working outside the home. It’s a day for the rest of the family to appreciate service often taken for granted.

Moms welcome a one-day breather from their multitasking routine. Modern conveniences such as washing machines and vacuum cleaners may make housework less of a chore, and new technologies such as computers and DVDs can help keep kids educated and entertained, but expectations for parenting have never been higher. Feeding, washing, and keeping the kids warm isn’t nearly enough. Modern mothers are supposed to cultivate young minds, read frequently to their newborns, help children explore their talents, and above all protect them from all the dangers of the modern world.

It’s a task list that makes even the most ambitious, determined women feel inadequate. A just-released collection of essays, Torn: True Stories of Kids, Career & the Conflict of Modern Motherhood, edited by Samantha Parent Walravens, provides a window into the struggles that today’s moms face. Some stories are humorous, others poignant, but they share a common theme: Today’s moms strain under the pressure and worry they aren’t doing enough for their kids.

Yet there’s good news for these mothers, and the millions of others who will relate to them. Another new book, this one from George Mason University’s Bryan Caplan, suggests that mothers (as well as fathers) deserve more than a one-day reprieve from the grueling schedule of modern parenting. Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think details the findings of studies of twins, which reveal the profound power of nature, and comparatively modest impact of nurture.

Caplan explains: “practically everything-health, intelligence, happiness, success, personality, values, interests-is partly genetic. The evidence is straightforward: Identical twins are more similar than fraternal twins in almost every way-even when the twins are separated at birth. But twin research has another far more amazing lesson: With a few exceptions, the effect of parenting on adult outcomes ranges from small to zero. Parents change kids in many ways; the catch is that the changes fade out as kids grow up. By adulthood, identical twins aren’t slightly more similar than fraternal twins; they’re much more similar. And when identical twins are raised apart, they’re often just as similar as they are when they’re raised together.”

As a mother of three, I found this book challenging. I’ve invested a lot of time since becoming a mom trying to cultivate my children’s intellect, through plentiful reading, stimulating extracurricular activities, and by strictly limiting exposure to television. Has this all been a waste? How should a mother feel about evidence that suggests that all her sacrifice has almost no measurable effect on her kids’ life prospects?

In short, mothers should feel relieved. Yes, it’s a little disappointing to have shouldered needless stress for years, but in economic terms, those are sunk costs. The more important question is how you will behave today, tomorrow, and for the years that remain before your children grow up and leave you.

There are important caveats in Caplan’s work: The research he cites focuses on mainstream families in Western societies, and therefore measures the effects of generally acceptable parenting strategies. Dire poverty, abuse or neglect would undoubtedly do lasting damage.

Yet for the many moms out there, sacrificing their sanity to try to give their kids a boost, the lesson is this: We can all relax a little bit. Of course, lots of work still needs to be done, but a priority should be enjoying the experience of parenting, engaging in activities that build your relationship with your child, rather than that attempting to mold him or her in preparation for adulthood.

Caplan’s economic take leads to another conclusion: Having kids is less costly than you currently think, so you might want to consider having more.

Mothers will appreciate the breakfasts in bed and flowers this Sunday. Yet the best gift will be one that only they can give themselves: a new outlook on parenting, as less of a chore and more of a joy.