Progress Daily

Girls who grow up without their fathers at home reach sexual maturity earlier than girls whose fathers live with them (and early-bloomers are more likely to suffer depression, hate their bodies, engage in risky sex and get pregnant in their teen years).

Research by Jane Mendle (”Associations Between Father Absence and Age of First Sexual Intercourse“) suggests heredity is the cause: the genes that make a dad more likely to leave his family also cause early sexual development.

The researchers analyzed data American National Longitudinal Survey of Youth data on 1,400 boys and girls, each of whom was related to at least one other subject through their mother. Most of the mothers were pairs of sisters, but some were identical twins or first cousins raised as sisters.

The more closely related the cousins were — by having mothers who were identical twins, for instance, versus cousins — the closer their age at first sexual experience, regardless of whether or not a father lived in the home.

“Daddy’s girl,” The Economist

In a study just published in Evolution and Human Behavior (”Costs and benefits of fat-free muscle mass in men: relationship to mating success, dietary requirements, and native immunity“), Steven Gaulin analyzes muscularity.

The data came from the NHNES, which followed 12,000 American men and women over the course of 6 years. The researchers found that men require 50% more calories than women do, even after adjusting for activity levels, and that their muscle mass is the strongest predictor of their intake of calories — stronger than their occupation or their body-mass index. Men’s immune systems are less effective than those of women (which was known before), and become worse the more muscular the men are (which was not).

The more muscular a man, the more sexual partners he reported, both in the past year and over his lifetime, and the earlier his first sexual experience was likely to have been.

Gaulin speculates that an evolutionary fight is going on between natural selection, which conserves metabolic expenditure and promotes longevity, and sexual selection, which willingly trades both for extra mating opportunities. This may explain why men have such a range of muscularity. In the past, the strong man would have had better mating opportunities in the short term, but the skinny guy who outlived him could have had just as much reproductive success over the course of his longer life.

“Mr Muscle,” The Economist

According to research by Hasse Walum (”Genetic variation in the vasopressin receptor 1a gene associates with pair-bonding behavior in humans“), men with a particular variant of a gene that influences brain activity (specifically, an allele that regulates the activity of the hormone vasopressin) are less likely to be devoted and loyal husbands.

About 40% of men have one or two copies of the allele. Men with two copies of the allele had a greater risk of marital discord than men with one copy, and men with one copy of the allele were at more risk of such discord than men with no copies.

In a sample of more than 1,000 heterosexual couples, about 15% of the men without the allele reported serious marital discord in the past year, compared with 34% of men with two copies of the allele.

17% of the men without the allele were living with women without being married to them, compared with 32% of men with two alleles.

“Study Links Gene Variant in Men to Marital Discord” by Shankar Vedantam

Wen Zhou & Denise Chen speculated that if humans produce and respond to sweat pheromones, then a woman should respond to a guy’s sweat when sexually aroused differently than she does to his normal sweat.

The researchers asked 20 heterosexual guys to put pads in their armpits as they watched pornographic videos and became aroused (verified by electrodes). Later, they collected the sweat they produced when they weren’t aroused. (See “Encoding Human Sexual Chemosensory Cues in the Orbitofrontal and Fusiform Cortices“)

The researchers then recruited 19 women to smell the men’s pads while undergoing (fMRI) brain scans. The sexual sweat, but not the normal sweat, activated the right orbitofrontal cortex and the right fusiform cortex, brain areas that help us recognize emotions and perceive things, respectively. Both regions are in the right hemisphere, which is generally involved in smell, social response, and emotion.

“Women Can Smell a Man’s Intentions,” Melinda Wenner

 Genes that make some people gay may make their siblings fecund.

Homosexuality is at least partly genetic. Studies of identical twins, for example, show that if one of a pair (regardless of sex) is homosexual, the other has a 50% chance of being so, too. How could a trait so at odds with reproductive success survive natural selection?

Brendan Zietsch, of the QIMR, thinks that genes which cause men to be more feminine in appearance, outlook and behavior and those that make women more masculine in those attributes, confer reproductive advantages as long as they do not push the individual possessing them all the way to homosexuality.

Gay men tend to rank higher than straight men in standardised tests for agreeableness, expressiveness, conscientiousness, openness to experience and neuroticism. Lesbians tend to be more assertive and less neurotic than straight women.

Though women prefer traditionally macho men at the time in their menstrual cycles when they are most fertile, at other times they are more attracted to those with feminine traits such as tenderness, considerateness and kindness, as well as those with feminised faces. This suggests that women prefer macho men for breeding but the more feminised phenotype for carers and providers (husbands). And husbands father most of the world’s children.

Zietsch’s researchers asked 4,900 individual twins, not all of them identical, to fill out anonymous questionnaires about their sexual orientation, their gender self-identification and the number of opposite-sex partners they had had during the course of their lives.

They found that the more feminine a man, the more masculine a woman, the higher the hit rate with the opposite sex — though women of all gender identities reported fewer partners than men did (which may reflect male boasting &/or female bashfulness).

Heterosexuals with a homosexual twin tend to have more sexual partners than heterosexuals with a heterosexual twin. The researchers then analyzed the relationships between twins (all genes in common for identical twins; a 50% overlap for the non-identical) and calculated that genes explain 27% of an individual’s gender identity and 59% of the variation in the number of sexual partners. (They also calculated that the genetic component of sexual orientation was 47% — similar to previous studies

“Gender bending,” The Economist

As human societies grow richer, people have fewer children. In most species, such an increase in available resources leads in the opposite reproductive direction. What makes the “demographic transition” even more paradoxical is that in less developed times and places, the rich do not have smaller families than the poor.

Most explanations of the demographic transition are social, and none is really satisfactory.

A study by Agnar Helgason, of deCODE Genetics, has recently provided a new explanation: that the mixing-up of people caused by the urbanisation which normally accompanies development is, itself, partly responsible — because it breaks up optimal mating patterns.

Iceland’s records since its founding by a few Vikings are so good that the antecedents of today’s inhabitants (apart from a handful of recent immigrants) are known with precision. Its medical records are also good, and most Icelanders have given genetic samples to deCode.

The study’s principal finding is that the most fecund marriages are between distant cousins. The optimum degree of outbreeding (measured in terms of the number of children and grandchildren produced) lay somewhere between cousins of the third and fourth degrees.

(”Kissing cousins, missing children,” The Economist)

Icelandic women born between 1800 and 1824 who mated with a third cousin had significantly more children and grandchildren (4.0 and 9.2, respectively) than women who hooked up with someone no closer than an eighth cousin (3.3 and 7.3). Those proportions held up among women born more than a century later when couples were, on average, having fewer children.

Despite the general pattern for reproductive success favoring close kinship, couples that were second cousins or more closely related did not have as many children.

With close inbreeding — between first cousins — there is a significant increase in the probability that both partners will share one or more detrimental recessive genes, leading to a 25 percent chance that these genes will be expressed in each pregnancy.

Mating with a relative might reduce a woman’s chance of having a miscarriage caused by immunological incompatibility between a mother and her child. Some individuals have an antigen (a protein that can launch an immune response) on the surface of their red blood cells called a rhesus factor. In some cases — typically during a second pregnancy — when a woman gets pregnant, she and her fetus may have incompatible blood cells, which could trigger the mother’s immune system to treat the fetus as a foreign intruder, causing a miscarriage. This occurrence is less probable if the parents are closely related, because their blood makeup is more likely to match.

It may be that the enhanced reproductive success at the level of third and fourth cousins (who on average would be expected to have inherited 0.8 percent to 0.2 percent of their genes from a common ancestor) represents a point of balance between the competing advantages and disadvantages of inbreeding and outbreeding.

(”When Incest Is Best,” by Nikhil Swaminathan)

Dr Randy Thornhill manipulated pictures to make people’s faces appear more and less symmetrical, then asked volunteers of the opposite sex rank them for attractiveness. Symetery and attractiveness correlated. His later experiments have shown that all aspects of bodily symmetry contribute, down to the lengths of corresponding fingers, and that the assessment also applies to those of the same sex.

Perfect symmetry is hard for a developing embryo to maintain, so one that can maintain it must have good genes (and luck).

Other aspects of beauty, too, are indicators of health. Skin and hair condition are sensitive to illness and malnutrition.

Leslie Zebrowitz and Gillian Rhodes found 9 past studies on attractiveness and IQ, and subjected them to a “meta-analysis.”

The studies’ researchers had photographed people and asked them to do IQ tests, then showed the photographs to other people and asked them to rank the intelligence of the first lot. The results suggested that people get such judgments right often enough to be significant.

Dr Daniel Hamermesh presided over a series of surveys in the USA and Canada that showed that when all other things are taken into account, ugly people earn less than average incomes, while beautiful people earn more than the average. The ugliness “penalty” for men was -9% while the beauty premium was +5%. For women, the ugliness penalty was -6% while the beauty premium was +4%.

He found the figures for men in Shanghai are –25% and +3%; for women they are –31% and +10%. In Britain, ugly men do worse than ugly women (-18% as against -11%) but the beauty premium is the same for both (+1%).

Dr Hamermesh found that those members of a particular (anonymous) US law school rated attractive on the basis of their graduation photographs went on to earn higher salaries. Moreover, lawyers in private practice tended to be better looking than those working in government departments.

Hamermesh’s study of Dutch advertising firms showed that those with the most beautiful executives had the largest size-adjusted revenues — a difference that exceeded the salary differentials of the firms in question. Finally, he found that attractive candidates were more successful in elections for office in the American Economic Association.

Working in Shanghai, where his research indicated the difference between the ugliness penalty and the beauty bonus was greatest, Dr Hamermesh looked at how women’s spending on their cosmetics and clothes affected their income.

The beauty premium generated by such primping was worth only about 15% of the money expended.

Niclas Berggren’s research team looked at almost 2,000 candidates in Finnish elections. They asked foreigners (mainly Americans and Swedes) to examine the candidates’ campaign photographs and rank them for beauty. The more beautiful candidates, as ranked by people who knew nothing of Finland’s internal politics, tended to have been the more successful — the effect was larger for women than for men.

“To those that have, shall be given,” The Economist

For a couple of years, researchers (Ray Fisman, Sheena Iyengar, Itamar Simonson, and Emir Kamenica) ran a speed-dating experiment at a local bar just off the Columbia campus.

Subjects were asked to rate their partners’ intelligence, looks, and ambition after each meeting. Each event had between 10 and 20 daters of each gender, and in the course of the evening, every man met every woman and vice versa.

The researchers collected data on thousands of decisions made by more than 400 daters from Columbia University’s graduate and professional schools.

They found that men put significantly more weight on their assessment of a partner’s beauty, when choosing, than women did.

Intelligence ratings were more than twice as important in predicting women’s choices as men’s. Men tended to prefer women whom they rated as relatively smart and ambitious but they avoided women whom they perceived to be smarter and more ambitious than themselves.

When women were the ones choosing, the more intelligence and ambition the men had, the better.

Women of all races (white, black, East Asian, and Hispanic) revealed a strong preference for men of their own race. But a woman’s race had little effect on the men’s choices.

The researchers found no evidence of a white male preference for East Asian women. But did find that East Asian women did not discriminate against white men (only against black and Hispanic men). So the white man-Asian woman pairing was the most common form of interracial dating. Daters of both sexes from south of the Mason-Dixon Line revealed much stronger same-race preferences than Northern daters.

“An Economist Goes to a Bar,” by Ray Fisman

Women — especially in the developed world — are getting married and having kids later than ever before. According to the UN’s World Fertility Report, the worldwide median age of marriage for women is up two years, from 21 in the 1970s to 23 today. In the developed countries, the rise has been from 22 to 26.

In 1960, 70% of American 25-year-old women were married with children. In 2000, only 25% were. In 1970, just 7% of all American 30- to 34-year-olds were unmarried. Today, the number is 22%. In today’s Hungary, 30% of women in their early thirties are single, compared with 6% of their mothers’ generation at the same age. In South Korea, 40% of 30-year-olds are single, compared with 14% only 20 years ago.

Between 1960 and 2000, the percentages of 20-, 25-, and 30-year-olds enrolled in school more than doubled in the US, and enrollment in higher education doubled throughout Europe. The majority of college students are female in the US, UK, France, Germany, Norway, and Australia, and the gender gap is quickly narrowing in more traditional countries like China, Japan, and South Korea. In Denmark, Finland, and France, over 1/2 of all women between 20 and 24 are in school.

In the UK, close to 1/3 of 30-year-old college-educated women are unmarried. In Spain, women now constitute 54% of college students, up from 26% in 1970, and the average age of first birth has risen to nearly 30, which may be a world record.

In the US, the proportion of unmarried 20-somethings living with their parents has declined steadily over the last 100 years, despite rising rents and real estate prices.

A 2005 report from MasterCard finds that women take 4 out of every 10 trips in the Asia-Pacific region — up from 1 in 10 back in the mid-’70s.

Canadian single women are buying homes at twice the rate of single men. The National Association of Realtors reports that in the US last year, single women made up 22% of the real-estate market, compared with only 9% for single men. The median age for first-time female buyers: 32.

Between 1994 and 2004, the number of Japanese women between 25 and 29 who were unmarried soared from 40% to 54%. The number of 30- to 34-year-old females who were unmarried rose from 14% to 27%.

A majority of Japanese single women between 25 and 54 say that they’d be just as happy never to marry.

Under European Communist rule, women tended to marry and have kids early. In the late ’80s, the mean age of first birth in East Germany, for instance, was 25, while the West German average was 28. Tying the knot was the only way to gain independence from parents, since married couples could get an apartment, while singles could not. Furthermore, access to modern contraception, which the state proved either unable or unwilling to produce at affordable prices, was limited. Marriages frequently began as the result of unplanned pregnancies.

Many towns in what used to be East Germany now face a lack of women, as women who excelled in school have moved west for jobs. In some towns, the ratio is just 40 women to 100 men. Women constitute the majority of both high school and college graduates in Poland.

Save Albania, no European country stood at or above replacement levels (2.1 children) in 2000. Three-quarters of Europeans now live in countries with fertility rates below 1.5, and even that number is inflated by a disproportionately high fertility rate among Muslim immigrants. Oddly, the most Catholic European countries — Italy, Spain, and Poland — have the lowest fertility rates, under 1.3. In Japan, fertility rates are about 1.3. Hong Kong, at 0.98 has broken the barrier of one child per woman.

With fewer children, the labor force shrinks, and so do tax receipts. Europe today has 35 pensioners for every 100 workers. By 2050, those 100 may be responsible for 75 pensioners.

By large margins, surveys suggest, US women want to marry and have kids. US fertility rates are lower than replacement level among college-educated women, but are still higher than those of most rich countries (including Sweden and France).

Young women in Vietnam have begun putting off marriage and fertility rates have fallen from 3.8 children in 1998 to 2.1 in 2006.

Fertility rates have dropped below replacement level in several of India’s major cities.

“The New Girl Order,” by Kay S. Hymowitz

The largest-ever global study of abortion (”Abortion: Worldwide Levels and Trends“) has found that restricting abortions has little effect on the number of pregnancies terminated. Rather, it drives women to seek illegal, often unsafe backstreet abortions leading to an estimated 67,000 deaths a year. A further 5m women require hospital treatment as a result of botched procedures.

In Africa and Asia, where abortion is generally either illegal or restricted, the abortion rate in 2003 was 29 per 1,000 women aged 15-44. This is almost identical to the rate in Europe — 28 — where legal abortions are widely available. Latin America, which has some of the world’s most restrictive abortion laws, is the region with the highest abortion rate (31), while western Europe, which has some of the most liberal laws, has the lowest (12).

A woman’s likelihood of having an abortion is similar whether she lives in a rich country (26 per 1,000) or a poor or middle-income one (29).

The same point can be made by looking at those countries which have changed their laws. Between 1995 and 2005, 17 nations liberalised abortion legislation, while three tightened restrictions. The number of induced abortions nevertheless declined from nearly 46m in 1995 to 42m in 2003, resulting in a fall in the worldwide abortion rate from 35 to 29. The most dramatic drop — from 90 to 44 — was in former communist Eastern Europe, where abortion is generally legal, safe and cheap. This coincided with a big increase in contraceptive use.

According to a report published this month by Population Action International women in poor countries are 250 times more likely to die in pregnancy or childbirth than women in rich ones.

A woman in Africa has a one in 16 chance of dying in pregnancy or childbirth, compared with one in 3,800 for a woman in the rich world.

“Safe, legal and falling,” The Economist