Sunday, April 6, 2014

The sex life of Charles Darwin


Charles Darwin's sex life is of interest because of his consanguineous marriage (to his first cousin), which seems to have resulted in genetic problems for his children, due to inbreeding (see Charles Darwin's family pedigree network). The children of this marriage have recently been discussed in the book by Tim Berra (Darwin and His Children: His Other Legacy). This book discusses Darwin's children mainly in the context of Darwin's own life. Unfortunately, it does not delve much into his personal relationship with either them or his wife, Emma. His private life remains fairly private.

In particular, the book fails to draw any inference from the obvious fact that there were 10 of these children, plus two possible miscarriages. However, obviously we do learn indirectly about a certain part of Mr Darwin's private life. After all, one does not get a woman pregnant accidentally (no matter what your friends try to tell you) -- there are certain biological procedures that you need to go through, and it is fairly difficult to carry these out accidentally. Clearly, Charles and Emma were familiar with this particular activity, and carried it out successfully on numerous occasions.

Charles Darwin, 2 years before the
birth of his last child

The question is: how many occasions? We know the minimum number, but what about the average rate, for example? The Darwin cottage industry has apparently produced speculations about his sex life before (see Wikipedia), but I have not read about them. Instead, I will provide my own analysis of the situation.

Background

Charles and Emma married on 29 January 1839, when Charles was 29 years and 11 months old and Emma was 30 years and 8 months old. This is pretty late to be starting a family, although not necessarily unusual, and it does have an influence on the calculations.

Emma realized during the following April that she was pregnant (ie. within 3 months); and during the subsequent 18 years she was pregnant a total of 11 more times. On average, there were 500 days between each of the first nine pregnancies, as shown in the first graph. This means that during those 12 years she spent 55% of her days being pregnant and 45% of them not pregnant.


Wikipedia paints an interesting picture of marriages in Victorian Britain (Women in the Victorian era):
When a Victorian man and woman married, the rights of the woman were legally given over to her spouse. Under the law the married couple became one entity where the husband would represent this entity, placing him in control of all property, earnings and money. In addition to losing money and material goods to their husbands, Victorian wives became property to their husbands, giving them rights to what their bodies produced: children, sex and domestic labour. Marriage abrogated a woman's right to consent to sexual intercourse with her husband, giving him 'ownership' over her body. Their mutual matrimonial consent therefore became a contract to give herself to her husband as he desired.
The extent to which Emma was involved in the decision to spend more than half of her time pregnant is therefore open to debate. Both her letters and those of her husband do not, as far as I know, reveal any marital difficulties — indeed, quite the contrary. However, Charles' has left us written evidence of his pre-marital ideas about marriage (Darwin’s notes on marriage), which indicate his specific intention to have a family available in his old age.

Note that there are reported to have between two miscarriages between the 9th and 10th births, one in 1852 (when Emma was 44 years old) and one in 1854 (when she was 46). Emma was 48 years and 7 months old when she delivered her final child. Along with the miscarriages, it is worth noting that the final child was born mentally disabled (probably Down's syndrome, for which there is a 1 in 11 chance at age 49), and he died after 18 months. Also, the third child was born after only 36 weeks of pregnancy (instead of the "normal" 40 weeks), and lived for less than a month. Darwin's favorite child was his 2nd (Anne), who unfortunately died of tuberculosis at age 10. The remaining seven children survived to adulthood.

We can also note that the children were born during most periods of the year, as shown in the next graph. However, five of the births were during the 3-month period from early July to late September, implying conception during the period October to December.


In English-speaking countries there is a peak of births in late September, 9 months after the Christmas celebrations (Wellings et al. 1999; Tita et al. 2001). (In Scandinavia, the birth peak is 9 months after the mid-summer celebrations.) Given that two of the births were in this period, we might accuse the Darwins of fitting into this behavioral cliché. However, one of the these two births was the shortened pregnancy, so that conception in that case was on or near to their 3rd wedding anniversary, rather than Christmas. The other conception dates do not fit any pattern that I can see.

All of the above data lead me to the conclusion that most, if not all, of the pregnancies were the result of more-or-less continuously ongoing sexual activity, rather than being the result of deliberate attempts to conceive, or being incidental by-products of celebratory activity. That is, the pregnancies occurred as chance dictated, given the night-time activities being undertaken.

This leads us to the key question of how often these activities took place. We can do some general calculations that might be informative.

Calculation

We now know that the potentially fertile period of human female ovulation is 12 days out of every 28, and vaginal sex during this period should be avoided if you do not wish to be involved in a pregnancy (Arévalo et al. 1999). Within this window of opportunity there is a 6-day period during which conception is most likely (Dunson et al. 2002; Stirnemann et al. 2013), and if you are trying to conceive a child then sex at least twice during this period is the recommended strategy. (Each egg lasts 1 day, but sperm last for 3 days, so that sex more than 2-3 times doesn't seem to improve your chances.) Clearly, sex once during this 6-day period is a reasonable minimum expectation for conception.

However, the probability of conception even under these minimum circumstances is very dependent on the age of the female involved. (The eggs are produced early in the female's life, and the eggs age along with the woman, so that older eggs have reduced fertility; Broekmans et al. 2009.) For example (Siebler 2009; Sozou & Hartshorne 2012), in her early 20s a healthy fertile woman has a 20–25% probability of conception each month. The average time to achieve conception for this age group is 4 months, and the likelihood of conception within one year is 93–97%. More importantly, in her early 30s (as Emma was when she married) the probability of conception each month drops to 10–15%, so that the average time of conception is 10 months and the likelihood of conception within one year is c.72%. The probability keeps dropping until menopause (where it reaches zero), so that, for example, the likelihood of conception within one year is c.65% for a woman in her late 30s.

Emma, near the time of her marriage

This means that, given her age, Emma had to receive sperm during every ovulation cycle, in order to maintain a 50% chance of getting pregnant within any one year (she got pregnant on average every 9-12 months). If you know the ovulation times, then that rate requires sex 13 times per year. If you don't know the times, or you don't know anything about ovulation cycles (and it seems likely that Victorian women did not), then it requires sex at least once per week in order to hit them all by random chance.

So, I arrive at the conclusion of weekly sex for the Darwins throughout the first 12 years of their marriage, and possibly for 18 years. Calculations seem to be much more difficult after that, due to lack of suitable data.

I have no idea whether this weekly rate was normal for Victorian couples, but it certainly seems to be quite normal in the modern world, for people of their age. As shown in the next graph, people in their 30s and 40s currently report having sex every 4-5 days throughout the year (Mosher et al. 2005; Schneidewind-Skibbe et al. 2008). So, Charles' sex life would fit perfectly into the 21st century.

From Mosher et al. (2005)

Postscript

Finally, it is interesting to note that Charles started writing what he called his "Big Species Book" shortly after the birth of his final child. Furthermore, he converted this incomplete manuscript into what is now known as On the Origin of Species after the early death of that same child. Other events were involved in these decisions, of course, but his changing family life is unlikely to have been the least important of them.

References

Arévalo M, Sinai I, Jennings V (1999) A fixed formula to define the fertile window of the menstrual cycle as the basis of a simple method of natural family planning. Contraception 60: 357-360.

Broekmans FJ, Soules MR, Fauser BC (2009) Ovarian aging: mechanisms and clinical consequences. Endocrine Reviews 30: 465-493.

Dunson DB, Colombo B, Baird DD (2002) Changes with age in the level and duration of fertility in the menstrual cycle. Human Reproduction 17: 1399-1403.

Mosher WD, Chandra A, Jones J (2005) Sexual behavior and selected health measures: men and women 15–44 years of age, United States, 2002. Advance Data From Vital and Health Statistics 362. National Center for Health Statistics, Hyattsville, MD.

Schneidewind-Skibbe A, Hayes RD, Koochaki PE, Meyer J, Dennerstein L (2008) The frequency of sexual intercourse reported by women: a review of community-based studies and factors limiting their conclusions. Journal of Sexual Medicine 5: 301-335.

Siebler SJ (2009) How to Get Pregnant. Little, Brown and Co, New York, NY.

Sozou PD, Hartshorne GM (2012) Time to pregnancy: a computational method for using the duration of non-conception for predicting conception. PLoS One 7: e46544.

Stirnemann JJ, Samson A, Bernard JP, Thalabard JC (2013) Day-specific probabilities of conception in fertile cycles resulting in spontaneous pregnancies. Human Reproduction 28: 1110-1116.

Tita AT, Hollier LM, Waller DK (2001) Seasonality in conception of births and influence on late initiation of prenatal care. Obstetrics & Gynecology 97: 976-981.

Wellings K, Macdowall W, Catchpole M, Goodrich J (1999) Seasonal variations in sexual activity and their implications for sexual health promotion. Journal of the Royal Society of Medicine 92: 60-64.

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