When men were boys – demographic data shows male puberty starts 4 years sooner than it did in 1800

You’ve no doubt heard that girls are entering puberty earlier than they ever have before. It’s one of those science-lite stories beloved of tabloid papers and current affairs programs. One recent article had 10 to 15 percent of American 7 year olds showing the first signs of breast development.

It’s one of those problems that has folks scratching around for an answer. Toxic chemicals in the environment, including those that fake the effect of hormones, often get the blame. Diet – specifically the overnutrition that’s also causing childhood obesity to balloon – is another popular culprit.

There are plenty of reasons to be concerned. Hormonal disruption can cause all sorts of diseases, including breast cancer. Likewise, obesity can lead to type II diabetes, coronary heart disease and many cancers.

Figure 1 from Goldstein's paper, shoing the 'accident hump' in male mortality.

Beyond these problems that might beset girls later in life, maturing too fast can make life difficult for little girls. Young girls can be notoriously precocious, and it isn’t hard to imagine that an 12-year old who looks 17 could find her world a bewildering and even dangerous place.

The start of breast development is thought to signal the beginning of puberty, and can be measured easily. Even better, the age at which a girl reports her first menstruation gives and unambiguous and easy to remember measure. And records go back a very long time, confirming that girls are experiencing menarche, on average, at younger ages.

But what about boys? It isn’t quite as easy to measure the onset of puberty in boys as it is in girls. An innovative new paper by Joshua R. Goldsteinat the Max Planck Institute for Demographic Research in Rostock, Germany, makes use of a rather unfortunate quirk of male demography to do so.

Both girls and boys experience declining mortality rates throughout childhood and early adolescence. For a brief period in the early teens, as girls enter menarche, their mortality rate slowly begins to rise, and it continues to do so gradually until around 50, at which time it takes off (see Figure 1).

About half a year after girls’ mortality begins to rise, boys also begin to die at much faster rates, and their chances of dying outstrip those of women until they reach about 30 years old.

This “mortality bump” is a famous feature of human demography. It occurs because pubescent and young adult boys suddenly become much more likely to die in accidents and violence. In a very short time, many boys go from relatively cautious children to wild thrill-seekers. Evolutionary psychologists reckon the burning desire to take risks comes from a furious jostling for respect and acceptance among male peers, and an urge to show off to girls.

Likewise, Martin Daly and Margo Wilson’s exceptional work on the causes of homicide shows that the most common form of killing, in which young men kill other, unrelated young men, tends to be the tip of an iceberg of simmering masculine rage. The urge to retain this respect and to compete with rivals can turn an otherwise innocent look or bump into a violent confrontation, some of which end in homicide.

Goldstein recognized that demographic records hold valuable information about the timing of male adolescence. Instead of finding out when each boy reached a particular landmark in development and then calculating the average age, he realized that the timing of the ‘accident hump’ could reveal the average age at which whole cohorts begin to behave like adolescents,

Figure 2 from Goldstein's paper, showing the decrease in age at which the accident hump manifests.

Sweden has kept fastidious demographic records for over 200 years, and several other countries have done so for almost as long. It turns out that over two centuries, young men have begun to mature a full four years earlier (see Figure 2).

Just as early pubescence in girls raises unique worries, so too we should  worry about ever younger men entering the period of anger, recklessness and relentless striving to impress peers and possible partners.

It is a problem that has long concerned Sir Peter Gluckman, the Chief Science Advisor to the New Zealand. He wrote a fabulous book, Mismatch: Why Our World No Longer Fits our Bodies, in which he discusses this and other ways in which our evolved bodies are no longer adapted to modern conditions.

Gluckman told the New Zealand news site stuff.co.nz that “MRI scanners have looked at the brain over time to see how it matures and several studies have shown quite dramatically that the last parts to mature deal with things like logic, judgement and wisdom. Those scans show they may not be mature until people are 25 to 30.”

While modern safety standards in developed nations probably make industrial and agricultural accidents as well as accidental drowning and homicide less likely than they were in 1811, we have invented plenty of new ways for young men to kill themselves since then. Foremost are personal firearms and that deadliest of weapons, the motor car.

Preview Sex, Genes & Rock ‘n’ Roll

If you’re curious and want to know whether it’s worth procuring a copy of Sex, Genes & Rock ‘n’ Roll: How Evolution has Shaped the Modern World (HINT: It Is), then here’s a preview, courtesy of Google Books.

If you like what you see, then there’s plenty of information here on where to buy the hardcopy, or an ebook for Kindle, iPad or other readers.

Cheap sex tests carry a hidden cost


This article was first published in the Sydney Morning Herald under the erroneous headline “Choosing baby’s gender threatens to skew society’s sex ratio”. (The article isn’t about “choosing gender” but rather about tests to find out the sex of a foetus that might make it easier for parents who only want a child of a particular sex to terminate a pregnancy, thereby choosing the sex of the child). As with any article, the comments ranged from thoughtful and considered to whacky.

Because the article concerns abortion, the comments often polarise pro-life and pro-choice, and some accuse me of being anti-abortion. I should specify up-front that I am solidly pro-choice. Like Bill Clinton, I tend to think “abortion should be safe and legal. It should also be rare.”

For folks who have a hard time understanding why rich people are more likely to conceive boys and poor people girls, this version provides links to some longer studies. I treat the subject at some length in Sex, Genes & Rock ‘n’ Roll.

Anyway, enough editorialising, here’s the article …………………

Continue reading Cheap sex tests carry a hidden cost

Dissecting Gender – Workshop at Melbourne Writer’s Festival

Rob is appearing at a workshop on Dissecting Gender as part of the 2011 Melbourne Writer’s Festival, on Sunday 28 August, 4-5pm. The other panellists are Jane McCredie and Cordelia Fine, and the session is to be moderated by Monica Dux.

The Event description says: Join Jane McCredie, Rob Brooks, Cordelia Fine and Monica Dux to consider the scientific evidence for there being differences between the male and female brain, how differences arise, how gender difference informs our culture, and whether it’s actually possible to get to the bottom of these questions.
The event is being filmed by the ABC’s Big Ideas program.
I believe I have two complementary tickets to the session, and I’m happy to give these to any Melbourne-based readers who are keen to come and be part of this event.

Review in The Sydney Morning Herald, Saturday 9 July 2011

The Sydney Morning Herald SmartEdition – SEX, GENES & ROCK’N’ROLL Rob Brooks New South, 303pp, $34.95 – 9 Jul 2011 – Page #37

Rob Brooks is professor of evolutionary biology at the University of NSW and believes that evolution is ‘‘the most important idea anybody ever had’’. In this highly enjoyable book he sets out to show how evolutionary biology can help in the understanding of the great issues of the 21st century.

Obesity, uncontrolled population growth, declining human fertility, global warming and other issues are all investigated through the prism of evolutionary biology and the results are invariably illuminating and controversial.


If you ever wondered why indigenous Australians battle with Type 2 diabetes and obesity then evolutionary biology has an answer. Did you know there is a reason why left-handed batsmen are so successful at international level and why ‘‘in developing countries, women tend to be at greater risk of obesity than men’’? This is a sublime piece of popular science.









If you can’t say “evolution” without choking on your own bile, then don’t lecture us about what is or isn’t “natural”.

Fred Nile isn't all that happy with Penny Wong. AAP Image/Dean Lewins

This article first appeared in The Conversation on 12 August 2011.

So Penny Wong, our finance minister, is going to be a mum. Sophie Allouache, Wong’s long-term partner, announced on Tuesday that her IVF-conceived baby will arrive in December.

While many commentators expressed appropriately genuine joy for the expectant couple, the news has also flushed out a predictable flow of concerned opposition from socially conservative quarters.

Predictably, Christian Democrat politician Fred Nile thinks it’s a bad idea. The list of things Fred Nile opposes is pretty long, but it features just about anything to do with homosexuality and children being raised by same-sex parents.

Not only does Nile oppose outright the idea of two mums raising a baby, but if they must do it he’d rather they kept their family life in the closet.

“She needn’t have made it public”, he told Melbourne’s Herald Sun. “It just promotes their lesbian lifestyle and trying to make it natural where it’s unnatural”.

Nile wants the Prime Minister to have a serious talk with Senator Wong, but suggests that the unmarried Julia Gillard lacks the proper moral authority, as she isn’t in a “traditional relationship” herself. Continue reading If you can’t say “evolution” without choking on your own bile, then don’t lecture us about what is or isn’t “natural”.