Friday, July 1st 2011
|Should I advise recently single ScarJo to stay
off the pill to find her next beau? From here.
Imagine you are a single, heterosexual woman. You meet a nice man at the driving range, or on a blind date. You like him and he likes you. You date, you get engaged, you get married. You decide to have a child together, so you go off the pill. One morning you wake up and look at your husband, and it’s like seeing him through new eyes. Who is this stranger you married, and what did you ever see in him?
After some articles made the news when they suggested mate preferences change on hormonal contraception, this seemed to be the scenario in the heads of many women. Is my pill deceiving me? What if my birth control is making me date the wrong man?
Several articles over the years have demonstrated that women prefer men with more masculine features at midcycle, or ovulation, and more feminine features in less fertile periods. Based on body odor, women and men also often prefer individuals with MHC (major histocompatibility complex) that are different from theirs, which may be a way for them to select mates that will give their offspring an immunological advantage. These findings have been replicated a few times, looking at a few different gendered traits. And as I suggested above, other work has suggested that the birth control pill, which in some ways mimics pregnancy, may mask our natural tendency to make these distinctions and preferences, regarding both masculinity and MHC (Little et al. 2002; Roberts et al. 2008; Wedekind et al. 1995).
On the one hand, I think it’s both interesting and important to consider the implications of the birth control pill beyond just contraception. Hormones are messages, so any cells that have receptors for these messages, like specialized mailboxes, can receive them. The pill is made of synthetic versions of estradiol and progesterone, and there are estradiol and progesterone receptors in your brain. And yes, these hormones do change your brain, both during the natural cycle and on hormonal contraception; Scicurious has written well on this in the past.
|Jolie, had she been on the pill and chosen her mate
differently. From here. Yes, looks to be a real pic.
On the other hand, I have a lot of questions: First and most important to me, how does any of this translate to non-straight women? I find the constant focus on mate choice between men and women a bit exhausting, and am not sure we can assume non-straight relationships to work the same way. Next, how well do preferences over the cycle map on to actual choices for mates, short term or long term? If we happen to find Brad Pitt more attractive than Justin Bieber at midcycle, does that mean no one will do but Brad Pitt? And finally, what are all the factors that we need to consider in mate choice besides a deep voice or square jawline (again, especially if you try to expand your thinking beyond straight relationships)?
I’ll start with the last two questions that deal with mate preference versus ultimate mate selection. As you all might expect, women and men choose mates for lots of reasons, not just masculinity or complementary immune systems. Bereczkei et al (1997) looked at singles ads and found women often sought mates with high parental care. In a separate singles ad evaluation, Pawlowski and Dunbar (1999) found that women mostly selected men of high resource potential who were interested in long-term relationships (either unlikely to divorce or unlikely to die within twenty years), where men selected women by markers of fecundity (ability to have babies). In a sample of 18-24 year old straight people in the US, Buston and Emlen (2003) found that most people selected mates who had similar characteristics to themselves. And a speed dating sample showed that people under those conditions selected dates based on easily observable traits, like physical attractiveness (Kurzban and Weeden 2005).
Now on to the fact that all of this research is on straight people. I found very little on lesbian women and the menstrual cycle… but what I found was very cool! Brinsmead-Stockham et al (2008) found that, like heterosexual women, lesbian women are quicker to identify unknown faces at midcycle, as long as they were the faces of the sex they preferred. So straight women were good at identifying male faces, lesbian women good at identifying female faces. Burleson et al (2002) found that sexual behavior in lesbian and straight women was mostly similar through the menstrual cycle, with both peaking at midcycle.
So, mate preference may be about telling a research assistant who is the hottest to you at a particular point in your cycle. And it is a fairly robust and consistent finding. However, when it comes to ultimate mate selection the most important thing to consider is a great point made by Pawlowski and Dunbar: finding a mate is about advertising what you have to offer while making known what you want in a mate. Then it’s all about finding some kind of compromise through a series of trade-offs based on what the individual wants, what they can offer, and what’s available in the dating pool. (So, since neither Brad Pitt nor Justin Bieber are currently in the dating pool, my previous comparison was pointless.)
Those of you who met your mate while on the pill: not to fear. I don’t think that the possibility that you may have some suppression of masculinized preferences at one point in your cycle means you’ve chosen the wrong person.
Who knows, it could have opened you up to the Mr. or Ms. Right.
Bereczkei T, Voros S, Gal A, & Bernath L (1997). Resources, attractiveness, family commitment; reproductive decisions in human mate choice. Ethology : formerly Zeitschrift fur Tierpsychologie, 103 (8), 681-99 PMID: 12293453
Brinsmead-Stockham K, Johnston L, Miles L, & Neil Macrae C (2008). Female sexual orientation and menstrual influences on person perception Journal of Experimental Social Psychology, 44 (3), 729-734 DOI: 10.1016/j.jesp.2007.05.003
Burleson MH, Trevathan WR, & Gregory WL (2002). Sexual behavior in lesbian and heterosexual women: relations with menstrual cycle phase and partner availability. Psychoneuroendocrinology, 27 (4), 489-503 PMID: 11912001
Buston PM, & Emlen ST (2003). Cognitive processes underlying human mate choice: The relationship between self-perception and mate preference in Western society. Proceedings of the National Academy of Sciences of the United States of America, 100 (15), 8805-10 PMID: 12843405
Kurzban R, & Weeden J (2005). HurryDate: Mate preferences in action Evolution and Human Behavior, 26 (3), 227-244 DOI: 10.1016/j.evolhumbehav.2004.08.012
Little AC, Jones BC, Penton-Voak IS, Burt DM, and Perrett DI. 2002. Partnership status and the temporal context of relationships influence human female preferences for sexual dimorphism in male face shape. Proceedings of the Royal Society of London Series B: Biological Sciences 269(1496):1095-1100.
Pawłowski B, & Dunbar RI (1999). Impact of market value on human mate choice decisions. Proceedings. Biological sciences / The Royal Society, 266 (1416), 281-5 PMID: 10081164
Roberts SC, Gosling LM, Carter V, & Petrie M (2008). MHC-correlated odour preferences in humans and the use of oral contraceptives. Proceedings. Biological sciences / The Royal Society, 275 (1652), 2715-22 PMID: 18700206
Wedekind C, Seebeck T, Bettens F, & Paepke AJ (1995). MHC-Dependent Mate Preferences in Humans Proceedings: Biological Sciences, 260 (1359), 245-249 DOI: 10.1098/rspb.1995.0087
Monday, June 6th 2011
Apologies for the re-post, it was the only way to save the post and comments with the correct tags after the Blogger meltdown the other day!
I’ve been accumulating a lot of mother-y links lately, thought I would share. First two Mother’s Day columns that remind us that we shouldn’t just put Mom on a pedestal and give her some chocolate one day a year, but think in a more systematic way about the oppression of women and children worldwide. Read this one by Esther Cepeda, and this one by Nicholas Kristof.
Women see Georgia O’Keefe art as erotic around ovulation. I’m not sure this really qualifies as evolutionary psychology, or needs that framework to understand that libido is higher near ovulation, which would increase the chances one would find erotic art extra erotic at that time.
Chimps give birth like humans. Very cool. I guess we didn’t notice until now because they are so solitary when they birth?
Cesarean sections are a major factor in maternal death. I don’t like how this article seems to blame the mother, given the way interventions seem to shunt many women towards C-sections whether they want one or not. But there are certainly many factors to consider in this issue, including the mother’s past health and the kinds of protocols used at the location where she is giving birth.
Cutting the cord too soon. This is an interesting piece in Time about the timing of cord clamping and its impact on respiratory issues in infants. Many birth centers and hospitals are advocating for a later time to clamp the cord for this and other reasons.
Mothering of all kinds
Hope for teenage mothers. This was a great story about a great program to help teen mothers have more success in school and beyond.
The amount of time a woman breastfeeds is related to her race and income. Not surprising, given that lactation support services are probably harder to come by, and that women who must earn an income can’t necessarily afford to go without pay for twelve weeks (that is the minimum maternity leave we get in the US, based on the Family Medical Leave Act, and most places give only that minimum). Even those women who do manage to get into a rhythm with breastfeeding lose it when they return to work, not just because of those short twelve weeks, but because few employers have workplaces set up for pumping.
Amy Poehler’s acceptance speech at the Time 100. She discusses the many other women (dare I say allomothers?) who support her as she raises her children and has a career. I may have teared up a little. Okay, I shut my office door and cried.
What measles vaccine refusal really costs. This is something parents should care about.
A hilarious account from a father about all the things you need to worry about — and expect to be judged upon — when having a child.
Finally, while this went around the interwebs when Dr. Isis wrote it the first time, re-read her AGORA post about why it’s all right to not be your mother.
The enduring gender gap in pay. Sigh.
Michele Bachelet should be everyone’s hero, if what I read in this story is any indication.
We can no longer escape the reality that BPAs (and other associated bisphenols, which unfortunately are what are being replaced in plastics that claim to be BPA-free) are endocrine disruptors that have negative consequences for health. Well, unless you’re Coca Cola. Then you are going to put your fingers in your ears and go “lalala!”
A lovely post on feminist reactions to street harassment. Another, very powerful read: kill me or leave me alone.
An important read about the use of language in journalistic storytelling, and the sexist way the New York Times originally covered the brutal gang rape of a little girl.
Historic STD posters. Were some sexist? Of course. But it only makes me want one for my office more, if for its ironic value.
Tuesday, May 17th 2011
Sons and daughters and differential parental investment
One of my favorite rhetorical tricks is asking my students a question that has an obvious answer based on cultural expectations, but is wrong. So every year, when I start to teach my students about parental investment, I ask:
Who is harder to raise, sons or daughters?
I’ve asked by a show of hands and with iClickers, over the years, and the room of 750 is almost unanimous: daughters are harder to raise. So, then I get off the stage and walk around a bit. What do you mean by that? I ask.
Girls cause more gray hairs.
Girls cause more trouble when they start to like boys.
Girls are more work, and cost more money, since they shop all the time.
Girls talk back more.
And of course, there is always the saying that girls steal some of their mother’s beauty.
So then I show them this:
|From Helle et al 2002.
Here is a graph of maternal longevity based on the number of sons or daughters they have. This data was based on a historical population from Finland from 1640-1870 using church records (Helle et al 2002). As you can see, the more sons mothers bear, the shorter their lifespans. You see the opposite for daughters. So sons have a negative impact, and daughters have a positive impact. This same trend has been found in records from a Flemish village (van de Putte et al 2003, 2004), where sons negatively impact lifespan but not daughters. Interestingly, data from church records from the field site where I work in rural Poland provides a slightly different picture: every offspring of either sex reduced lifespan by about 95 weeks (Jasienska et al 2006).
|From Jasienska et al 2006.
Once students see these graphs, they quickly realize what is going on. Generally speaking, girls help mothers more at home in terms of chores and alloparenting. And in many cultures, particularly the historical ones studied so far, sons are costly because parents invest more in them, to help launch their own families. Daughters, not so much. In the Polish population, there may be other factors where daughter investment is important, or it is just costly to have so many offspring and you have maternal depletion regardless of daughter help.
Sex bias in parental investment is an important part of understanding both the biology and culture of parenting, and the developmental trajectories of children. The Trivers-Willard hypothesis, which has been tested many times in humans and animals, suggests that parents should invest more in sons when conditions are good, and more in daughters when conditions are bad. That is, when you have lots of resource you should put it towards a son in order to increase the chances he will have high reproductive success, since his is assumed to be more variable and high effort could lead to high reward. But in periods of low resource, daughters are a good bet because they are more likely to have at least some reproductive success no matter what.
|From Hrdy 1990.
Of course, differential investment based on resources is further conflated in humans due to culture and, I would contend, our almost universal favor for patriarchy (Smuts 1995). Here is an image of an Indian family waiting at a clinic. There is a mother, an older son, and a twin boy and girl. Notice the extreme difference in health between the infant boy and girl – they are twins, yet the infant girl is emaciated. This is because in this population sons are always fed and cared for first, and whatever is left over, if there’s anything, is given to daughters.
So, parental investment can have real effects on the parent in terms of lifespan, and perhaps also their own future reproductive capabilities. Further, the conditions under which you may have children can vary, but how much a parent chooses to invest in their children varies too.
The piece of this that may be toughest to parse out, particularly in humans, is how the condition of the mother (or parents) can vary, and how that variation impacts the sex ratio of their children. In some species, like red deer, it is easier to imagine a mechanism: these animals have diapause, a period where their embryos are dormant until it is a good time to gestate and bear them. It is easier to insert some kind of selection process into a period where several embryos are all “frozen” and sex has been determined. But what about humans that produce singletons and invest huge, overlapping amounts of support to their children over decades? How would a sex bias based on maternal condition operate? And is there anything the offspring can do about it?
Changes in maternal breast size during pregnancy
It turns out that measurements as easy as stepping on a scale, and knowing your bra size, can begin to unpack the answer. First, a confession: I consider the author of this paper Andrzej Galbarczyk more than a colleague, but a friend. Andrzej is the graduate student who oversaw my Polish field site last season (Mogielica Human Ecology Study Site, director Dr. Grazyna Jasienska). He has translated consent forms and surveys for me and we’ve had many valuable and important conversations about my fieldwork. He is a smart, kind and thoughtful person and scholar. So, I let him see an early draft of this post to make sure I understood his point of view.
Galbarczyk performed an internet survey in Poland with 120 women, where he asked them to report their pre-pregnancy weight and bra size, their bra size directly after giving birth, and the sex of their offspring. He found two notable differences in these women: mothers of daughters weighed less before pregnancy, but had a greater changes in breast size during pregnancy.
The evidence about maternal pre-pregnancy weight is consistent with the Trivers-Willard hypothesis, as mothers who had sons were more likely to be heavier, and thus have more resource to invest. The second significant difference, that mothers with daughters had larger breasts after pregnancy, seems could be argued either way: Galbarczyk argues that it supports Trivers-Willard because mothers of sons could have been devoting more resource to growing their offspring rather than their breasts.
In other animals and primates particularly, mothers of male infants produce more energy-dense milk, yet mothers of female infants may produce a greater quantity of milk (Hinde 2009). And breast size is a pretty noisy signal of milk quality or quantity. So, what is the meaning of this difference in breast size?
Adaptation or physiological inevitability?
Galbarczyk suggests the difference is related to the evolutionary underpinnings of human female breasts. Women develop breasts around puberty, and though they certainly change in size and shape over time, keep them their whole lives. Other animals develop their mammary glands only shortly before lactating and then they regress again. Many contend that human breasts are an honest signal of fertility. This is at least partially confirmed by the correlation between breast size and estradiol concentrations (Jasienska et al 2006).
Galbarczyk thinks that the larger breasts seen in postpartum mothers to daughters may be a way to attract a mate for parental care. Perhaps this would help where she has given birth to the less-favored sex and needs to really convince him to participate; this could be a signal from the mother or the female fetus. Or maybe by appearing more attractive, she can have another reproductive opportunity, which would give her a chance to have a son.
You all know how I feel about evolutionary storytelling. In certain ways I do find this particular argument compelling, from the perspective of the Trivers-Willard hypothesis. But the evidence for the adaptive scenario around breast size is circumstantial.
Also, I don’t want this story to detract from some very interesting data: remember that Galbarczyk found that in this population, mothers of daughters weigh less before pregnancy, and develop larger breasts afterwards. Very cool. So perhaps we should consider a mechanistic, rather than adaptive explanation?
I have two thoughts about this, both related to androgens (androgens are the class of hormone that testosterone falls under). First, I wonder if there is an effect of fetal androgens from a male fetus on breast size. If so, mothers of daughters would have larger breasts simply because they aren’t having their breast tissue growth or density suppressed by androgens. It could simply be physiology that doesn’t have adaptive meaning.
Second, the mothers of sons were heavier before pregnancy. Heavier individuals tend to have higher circulating insulin levels, and the ovary can respond to higher insulin by producing more androgens (Poretsky 1991, Dimitrakakis et al 2004). So you could have a suppressive effect on breast size from that avenue as well. You don’t need an adaptive scenario for either of these mechanisms, just a consequence of how hormones work.
I would love to see Galbarczyk or someone else follow up on these thought-provoking results by measuring women, rather than relying on self-report, and by measuring their estradiol, progesterone and androgens. Understanding the different factors and motivations that lead to sex differential investment and outcome is a great field of study, and this work gets us thinking in a new direction.
Dimitrakakis C, Jones RA, Liu A, & Bondy CA (2004). Breast cancer incidence in postmenopausal women using testosterone in addition to usual hormone therapy. Menopause (New York, N.Y.), 11 (5), 531-5 PMID: 15356405
Galbarczyk A (2011). Unexpected changes in maternal breast size during pregnancy in relation to infant sex: An evolutionary interpretation. American journal of human biology : the official journal of the Human Biology Council PMID: 21544894
Helle, S. (2002). Sons Reduced Maternal Longevity in Preindustrial Humans Science, 296 (5570), 1085-1085 DOI: 10.1126/science.1070106
Hinde K (2009). Richer milk for sons but more milk for daughters: Sex-biased investment during lactation varies with maternal life history in rhesus macaques. American journal of human biology : the official journal of the Human Biology Council, 21 (4), 512-9 PMID: 19384860
Hrdy, S. (1990). Sex bias in nature and in history: A late 1980s reexamination of the “biological origins” argument American Journal of Physical Anthropology, 33 (S11), 25-37 DOI: 10.1002/ajpa.1330330504
Jasienska G, Nenko I, & Jasienski M (2006). Daughters increase longevity of fathers, but daughters and sons equally reduce longevity of mothers. American journal of human biology : the official journal of the Human Biology Council, 18 (3), 422-5 PMID: 16634019
Poretsky L, Seto-Young D, Shrestha A, Dhillon S, Mirjany M, Liu HC, Yih MC, & Rosenwaks Z (2001). Phosphatidyl-inositol-3 kinase-independent insulin action pathway(s) in the human ovary. The Journal of clinical endocrinology and metabolism, 86 (7), 3115-9 PMID: 11443175