## How often?

In my last post, I said that a wife should be willing to sleep with her husband once a month, and that this would fulfill the marriage debt.  Since I know that some of you plan your private lives around my advice, I’ve decided I owe it to you to do more than just pull a number out of the air.  Here’s why once a month is about right.

Suppose we want an average woman to have 3 children.  Ask.com tells me that the median marriage age for American women is around 25, and the probability of becoming pregnant from one sex act is around 5%.  Let’s say the woman stops having children at around 40.  That’s 15 child-bearing years; however, we must excuse the woman for the roughly two years covering her pregnancy and the child’s infancy, so we’re really left with 9 years to get pregnant three times, i.e. 3 years to get pregnant once.  Let’s say the couple has sex at time intervals dt, so that in 3 years, they have sex n=3/dt times.  The probability of not being pregnant in 3 years is (1-p)^n, where p=0.05.  Say we want to make this probability to be something moderately low, say 20%.  Then a trivial calculation gives

dt = 3 log(1-p)/log(0.2) = 0.096 = about 10 times per year

So my “once a month” rule of thumb is about right.

### 3 Responses

1. I have to disagree. At least once a week is my rule of thumb.

2. Does the marriage debt end when the woman either has given birth to three children or is past child bearing? Just curious.

And since marriage act is supposed to be an expression of love between spouses, it seems to me that reducing it to a “debt” is demeaning the act, making it an obligation rather than a expression of love and closeness between partners.

3. Dear Cindy,

Thank you for commenting on my post. I think your comment gets to the heart of the matter–is there an opposition between something being a duty and something being interpersonally meaningful? We often tend to speak like this when we say things like “Oh, that’s so touching because he didn’t have to do it,” or “He was obliged to do that, so it doesn’t really mean anything.” Still, I think the idea of an opposition is a mistake. Some of the greatest expressions of love come from fulfilling our duties. When a mother cares for her sick child, this is obviously both a duty and an act of love. We don’t make it more meaningful by denying its morally obligatory nature. When a parent lays down his life for his children, or a soldier for his country, they do no more than duty demands, but there is “no greater love than this…” Instead of opposing love to duty, we should recognize their organic relation. Binding oneself to another is love’s natural culmination, and recognizing another’s claim on one is a very interpersonally significant act.

Also, if the conjugal act is not a duty, then a woman could deny it to her husband all her life without moral fault, but this I deny. She has no right to deny her husband an heir without grave reason.

As for your other questions, I don’t believe that the tradition addresses them, but I would say yes, a woman who is infertile, past childbearing age, or already has several children has no further duties in this regard.