Why do beautiful people have more daughters? Because beauty is more important for a woman than a man, according to evolutionary psychologist Satoshi Kanazawa.
Why are most suicide bombers Muslim? Because they don't get enough sex. Why are liberals more intelligent than conservatives? Because liberalism is "evolutionarily novel."
The London School of Economics researcher and author of Ten Politically Incorrect Truths about Human Nature is accustomed to defending his provocative assertions against outraged critics.
He acknowledges that some of his ideas may seem "immoral, contrary to our ideals or offensive". But he insists they are true and supported by scientific evidence that he has continued to collect since his book was published in 2007. "Like it or not, human nature is simply not politically correct," he says.
Now, in a study to be published in Reproductive Sciences, he has adduced new evidence for what he describes as one of the most celebrated principles in evolutionary biology which explains why attractive people have more female children. So how does the research stack up?
1 Beautiful people have more daughters
Known as the Trivers-Willard hypothesis this states that if parents have any traits they can pass on to their children and that will be better for one sex than the other, they they will have more children of that sex.
A man's value as a mate is largely determined in evolutionary terms by his wealth, status and power, according to Dr Kanazawa, whereas a woman's is largely determined by her youth and physical attractiveness.
"Physical attractiveness, while a universally positive quality, contributes even more to women's reproductive success than to men's. The hypothesis would therefore predict that physically attractive parents should have more daughters than sons," Dr Kanazawa writes.
Figures from the 1958 National Child Development Study of 17,000 babies who were rated for attractiveness by their teachers at age 7 and were asked at age 45 for the age and sex of their babies bears this out. The unattractive children were more likely to have sons, according to the Reproductive Sciences study.
2 Liberals are more intelligent than conservatives
A study by Dr Kanazawa, published in Social Science Quarterly in March, based on the same data showed that young adults who identified themselves as "very liberal" had an average IQ of 106 while those who identified themselves as "very conservative" had an average IQ of 95.
"The ability to think and reason endowed our ancestors with advantages in solving evolutionarily novel problems for which they did not have innate solutions. As a result, more intelligent people are more likely to recognise and understand such novel entities and situations than less intelligent people, and some of these entities and situations are preferences, values, and lifestyles," Dr Kanazawa said.
Humans are evolutionarily designed to be conservative, caring mostly about their family and friends. Being liberal and caring about an indefinite number of genetically unrelated strangers is evolutionarily novel. So more intelligent children may be more likely to grow up to be liberals.
3 Most suicide bombers are Muslim
Suicide missions are not always religiously motivated. But when religion is involved it is always Muslim, Dr Kanazawa says.
Why? The surprising answer is that it may have nothing to do with Islam or politics, culture or race. Rather, it has to do with sex, or in this case the absence of sex.
Writing in Psychology Today, Dr Kanazawa said the distinguishing feature of Islam was that it tolerated polygyny – men taking two or more wives at the same time. By allowing some men to monopolise all women other men were left out. The prospect of 72 virgins waiting in heaven for any martyr to Islam then created a potent cocktail. "It is the combination of polygyny and the promise of a large harem of virgins in heaven that motivates many young Muslim suicide bombers," he says.
4 Men like blonde bombshells (and women want to look like them)
Blonde hair is unique in that it changes dramatically with age, Dr Kanazawa says. Typically, young girls with light blonde hair become women with brown hair. So blonde hair is a signal of youth and men who attempt to mate with blondes are unconsciously seeking younger (and hence healthier and more fecund) women.
5 Humans are naturally polygamous
Polyandry (one woman married to more than one man) is rare but polygyny has been widely practised throughout most of history.
In societies where rich men are much richer than poor men, women and their children are better off having a share of the few wealthy men than having an entire poor man to themselves. In practice, most industrial societies tend to be monogamous because men tend to be more equal in their resources than their ancestors in medieval times.
6 Having sons reduces the likelihood of divorce
A man's value as a mate is largely determined by his wealth, status and power whereas a woman's comes mainly from her youth and physical attractiveness. A father is important to his son in ensuring he inherits wealth status and power but he can do little to keep his daughter youthful or beautiful. His continued presence in the family is important to the son but not as crucial to his daughter.
7 What creative geniuses have in common with criminals
The tendency to commit crimes peaks in adolescence and then rapidly declines. But this curve is not limited to crime – it is also evident in every quantifiable human behaviour that is seen by potential mates and costly (not affordable by all sexual competitors). In the competition for mates men may act violently or they may express their competitiveness through their creative activities.
8 The myth of the male mid-life crisis
Many middle aged men go through a mid-life crisis but not because they are middle aged. It is because their wives are. Dr Kanazawa says: "From the evolutionary psychologist's perspective, a man's midlife crisis is precipitated by his wife's imminent menopause and end of her reproductive career and thus his renewed need to attract younger women."
9 It's natural for male politicians to risk everything for an affair
Powerful men have always married monogamously but mated polygynously. Men strive to attain political power, consciously or unconsciously, in order to have reproductive access to a large number of women.
10 Men sexually harass women because they are not sexist
Men always subjected each other to abusive, intimidating and degrading treatment at work. It is part of their reaction to competitive situations. Men are not treating women differently when they harass them. They do it because they are not discriminating, Dr Kanazawa says.