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

The cost of sequencing DNA has fallen from about $1 per base pair in the mid-1990s to a tenth of a cent today. Rob Carlson started tracking the price of DNA synthesis a decade ago. He found a steady decline, from over $10 per base pair to, lately, well under $1. This decline recalls Moore’s law, which, when promulgated in 1965, predicted the exponential rise of computing power.

A study by Amir Barnea, Henrik Cronqvist and Stephan Siegel (”Nature or Nurture: What Determines Investor Behavior?“), based on characteristics for approximately 40,000 identical and non-identical twins from the Swedish Twin Registry and associated investing data for the period 1998-2006, concludes that genetics explain up to 45% of individual variation in stock market participation, asset allocation and portfolio risk choices.

Genetic influence is robust to differences in age, education and net worth. Genetics explain more of the variation in individual investing behavior than does an extensive set of individual characteristics combined.

Family environment has an effect on the investing behavior of younger individuals, but this effect disappears as they acquire non-family experiences.

“The Genetics of Investing (Not the Algorithms),” CXO

Research by Paola Sapienza & Luigi Zingales (”Gender differences in financial risk aversion and career choices are affected by testosterone“) suggests that testosterone levels may explain why men dominate risky financial professions.

The researchers measured the amount of testosterone in the saliva of aspiring bankers (MBA students from the University of Chicago). They also estimated the students’ exposure to the hormone before they were born by measuring the ratios of their index fingers to their ring fingers (a long ring finger indicates high testosterone exposure) and by measuring how accurately they could determine human emotions by observing only people’s eyes, which also correlates with prenatal exposure to testosterone.

The students had to decide between a 50:50 chance of getting $200 or a gradually increasing sure payout, which ranged from $50 up to $120. The point at which a participant decided to switch from the gamble to the sure thing was reckoned a reasonable approximation of his appetite for risk.

Women and men with the same levels of testosterone generally switched at the same time, demonstrating similar risk preferences. Women who had more testosterone were more risk-loving than women with less, while the data for men at the lower end of the spectrum displayed a similar relationship. Curiously, the relationship between testosterone and risk taking was not as strong for men with moderate to high levels.

The correlation was strongest when the salivary measure of testosterone was used.

The researchers then followed the subjects’ progress after they graduated, to see what sort of careers they entered. Men were more likely than women to choose a risky job in finance but the difference was accounted for entirely by their levels of salivary testosterone.

“Risky business, The Economist

Non-kin altruism among non-human animals is rare, & altruism fares poorly in computer simulations — when altruistic individuals emerge in a community characterized by self-interested behavior, selfishness triumphs.

Group selection could explain the prevalence of human altruism but probably only if the ancestral environment included high levels of violence and inbreeding.

Sam Bowles showed in 2006 that genetic analyses of tribes still living a Stone Age life suggests there was enough inbreeding to make group competition a driver of genetic change.

In ancient graves excavated previously, Bowles found that up to 46% of the skeletons from 15 different locations around the world showed signs of a violent death. More recently, war inflicted 30% of deaths among the Ache, 17% among the Hiwi, & just 4% among the Anbara. Combat between groups accounted for about 14% of all deaths in these hunter-gatherer societies.

After estimating the rate that altruism would reduce an individual’s chances of reproducing, Bowles plugged the numbers into a model of intergroup competition where an individual’s altruism would also improve a group’s chances of combat triumph. In the absence of war, a gene imposing a self-sacrificial cost of as little as 3% in forgone reproduction would drop from 90% to 10% of the population in 150 generations. Bowles predicts that much higher levels of self-sacrifice — up to 13% in one case — could be sustained if warfare were brought into the equation.

“Did Warfare Among Ancestral Hunter-Gatherers Affect the Evolution of Human Social Behaviors?” By Samuel Bowles

“Altruism’s Bloody Roots,” by Brandon Keim

“Ancient warfare: Fighting for the greater good,” by Ewen Callaway

“Blood and treasure,” The Economist

Income is very heritable.

A study by Samuel Bowles & Herbert Gintis (”The Inheritance of Inequality,” 2002) found that identical twins’ incomes have a correlation of .56, versus .36 for fraternal twins. Using standard formulae, this implies that genetic factors explain 40% of the variance of income, family environment 16%, and non-shared environment 44%.

Using different data, David Cesarini gets income correlations of .54 for identicals versus .27 for fraternals, implying that genes explain 56% of the variance, shared environment -1%, and non-shared environment 45%.

“Is Greed in the Genes?,” by Bryan Caplan

Nicos Nicolaou’s research team used quantitative genetics techniques to compare the entrepreneurial activity of 870 pairs of monozygotic (MZ) and 857 pairs of same-sex dizygotic (DZ) twins from the UK, and found relatively high heritability for entrepreneurship, with little effect of family environment and upbringing.

Because MZ twins share all of their genetic composition and DZ twins share half of their genetic material on average, differences in the concordance between entrepreneurial activity of MZ and DZ twins can be attributed to genetic factors.

Studies of MZ and DZ twins raised together and apart have shown that the MZ twins raised apart are consistently more similar than DZ twins raised together.

Many parents are misinformed or make erroneous evaluations about the zygosity of their twins, leading some parents to raise their DZ twins as MZ twins and other parents to treat their MZ twins as DZ twins. Studies have shown that in cases where parents made erroneous conclusions about the zygosity of their twins, it was actual, rather than perceived zygosity that predicted similarity between twins.

The researchers estimate that 48% of the variance in the propensity to become self-employed is explained by genetic factors. None of the variance in self-employment can be attributed to shared environmental factors, while 52% can be attributed to non-shared environmental factors plus measurement error.

The researchers re-ran the analysis adjusting the model for potential confounders - income, education, marriage, age, race, and immigrant status - that have been shown to be associated with entrepreneurship. The heritability estimates for self-employment, drop only to 41% when these variables are included.

To identify the mechanism through which genetic factors operate, the researchers examined the respondents attitude toward entrepreneurship through an item that asked them to indicate, on a scale from one (a very good career) to five (a very bad career), their view of entrepreneurship as a career. The best fitting model to explain the variance in attitudes included shared and unshared environmental factors. None of the variance was explained by genetic factors, indicating that the mechanism through which these factors affect the tendency to become an entrepreneur is not through attitudes toward the vocation.

“Is the Tendency to Engage in Self-employment Genetic?” (.pdf file here), by Nicos Nicolaou, Scott Shane, Tim D. Spector, and others

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

People who are perceived to be trustworthy are more likely to have a higher credit score and pay lower interest rates on loans, and are less likely to default, according to a study by Jefferson Duarte & Stephan Siegel (”Trust and Credit“).

The researchers studied members of Prosper.com, where people looking for loans are matched up with individual lenders.

Each Prosper.com loan applicant submitted a profile which included credit and work history, education level, income and an optional photograph of themselves for lender review.

More than 6,800 loan applications, 2,600 loans and 12,000 photographs were used in the study.

Duarte hired a team of 25 people to rate the applicants’ trustworthiness on a scale of 1 to 5 using only the photographs of the borrowers. The team also judged the probability that the borrowers would repay a $100 loan.

Those judged to be trustworthy were more likely to get a loan from Prosper.com lenders and tended to have a credit score about 20 points higher than those determined to be untrustworthy.

“Untrustworthy” borrowers were 7% more likely to default on their loan than a perceived trustworthy borrower with the same credit score.

The researchers controlled for race, age, gender, obesity, attractiveness and education, employment status, income and homeownership.

“Creditworthiness may be linked to looks,” by Rebekah Kebede

The volume of the brain’s grey matter, made up of “processor” cells, is heritable and correlates with certain elements of IQ (Nature Neuroscience, DOI: 10.1038/nn758).

The amount of white matter, which provides the connections between these processors, is also heritable (Journal of Neuroscience, vol 26, p 10235).

And a new study suggests that the quality of these connections is also largely genetic, and correlates with IQ.

Paul Thompson scanned the brains of 23 sets of identical twins and the same number of fraternal twins, using HARDI (a type of magnetic resonance imaging).

By comparing brain maps of identical twins, which share the same genes, with fraternal twins, which share about half their genes, the team calculate that myelin integrity is genetically determined in many brain areas important for intelligence.

This includes the corpus callosum, which integrates signals from the left and right sides of the body, and the parietal lobes, responsible for visual and spatial reasoning and logic. Myelin quality in these areas is correlated with scores on tests of abstract reasoning and overall intelligence (The Journal of Neuroscience, vol 29, p 2212).

Myelin integrity is an especially promising target for manipulation, because, unlike the volume of grey matter, it changes throughout life.

Richard Haier said: “If it’s genetic, it’s biochemical, and we have all kinds of ways of influencing biochemistry.” 

“High-speed brains are in the genes,” by Aria Pearson