My source is this 2011 Newsweek article. The author (Niall Ferguson) is favorably citing Gunnar Heinsohn's "male youth bulge"^^1 theory. Here's the context:
The question left open by economists is what the consequences will be of such a large surplus of young men. History offers a disquieting answer. According to the German scholar Gunnar Heinsohn, European imperial expansion after 1500 was the result of a male “youth bulge.” Japan’s imperial expansion after 1914 was the result of a similar youth bulge, Heinsohn argues. During the Cold War, it was youth-bulge countries—Algeria, El Salvador, and Lebanon—that saw the worst civil wars and revolutions. Heinsohn has also linked the recent rise of Islamist extremism in countries like Afghanistan, Iraq, and Pakistan to an Islamic youth bulge. Political scientists Valerie Hudson and Andrea den Boer warn that China and India could be the next countries to overdose on testosterone.
I've seen this claim in several other contexts. For example, the the dangers of having too many unmarried men is cited as a reason not to allow polygyny. And there's this book, which I haven't read but apparently shows that "where there is violent and disorderly behavior, there are plenty of men, largely young and single." So it's not just Heinsohn and Ferguson.
For specificity, I'm only asking about European imperialism. In that case, the theory seems to bite off more than it can chew. Colonial expansion lasted 4 centuries, with most of Africa only getting claimed by the close of the 19th century.
Was there really an excess of young men in Europe 500 years ago, and how large was its role in subsequent empire-building?
(Second attempt; first attempt here)
^^1 Reminder: no top-level jokes, hard as it may be
follow-up: i've heard the same explanation for the routiers causing trouble in france centuries earlier; was there really a consistent imbalance in europe for such an extended period of time?
Ascribing the entirety of European Imperialism to a single cause is without a doubt stretching the utility of an idea too far. A period that can arguably be stretched into the 1950's and beyond is simply too broad to be given a single reason for, no matter how well it seems to fit. Additionally, that Heinsohn is actually advocating as such may be dubious (but more on that in a bit).
Heinsohn's theories, seem to be mired in controversy and his book 'Söhne und Weltmacht: Terror im Aufstieg und Fall der Nationen' is at present only in German. From what I can tell English speakers are mostly aware of him through a few online lectures and videos and this interview that has been translated to English. It's a wild ride, -some of which I'll address- but I think I may have located the statement Ferguson used for the 'cause of imperialism' remark
"Let us look at the small countries in Europe that were capable of conquering and colonising large parts of the world from around 1500, starting with Portugal and Spain. Our explanation is usually that there was a pressure on resources because of overpopulation. The opposite was the case. When Spain started its conquests in 1493 with Columbus’ second expedition, Spain had a population of six million, but in 1350 it had had nine million inhabitants. Spain was not overpopulated. There was, however, a sudden a growth in childbirths because in 1484 Pope Innocent VIII had decreed that birth control was punishable by death, which caused an immediate explosion in births. In the middle ages the average number of children per family was 2-3; now it was suddenly 6-7. That caused the median age in the population of six million to be 15, whereas the nine-million population of 1350 had had a median age between 28 and 30. So there was no lack of land or food. However, there was a sudden scarcity of positions. Previously there had been one or two boys in the family. One could take over the farm and the other might become a tenant somewhere else. Now you had three sons who had food but no positions, and these boys started the conquests and the colonising. It was quite telling that the Spaniards called then secundones, the second sons."
-- in reply to a somewhat spurious question about why the Middle East was 'underdeveloped' before the Europeans got there and what role religion played (he addresses religion but it's not really relevant to this)
Notice that Heinsohn's evidence is for a specific and short-term demographic shift, starting in 1484, which correlates only with the very beginning of the Colonial Period. I think the cause for Ferguson's statement is Heinsohn saying 'from around 1500' rather than specifying that this is a specific instance that would only partially explain the larger centuries-long developments later. Further examples Heinsohn provides in the interview were the rise of Hitler and Mussolini, which he ascribes to a youth bulge, but is careful to specify that the bulges were over by '39. From this, I think Heinsohn's Bulges are supposed to be thought of as temporary events only lasting a decade or two: the other examples Ferguson cites seem to corroborate this.
As to the actual academic value: I don't have access to enough resources or any of his research to properly gauge if the data he uses is good or not, but even assuming his numbers are in order there are reasons to question the conclusions he draws from them. For convenience, lets use the case I've already quoted about Portugal and Spain. The over-sea expansion of the Iberian states was already well underway without the bulge- Spain's conquest of the Canary Islands, starting in 1404, and Portugal rounded the Cape of Good Hope in 1488 (when the 'baby-boom' would be in its literal infancy). America was an unexpected and extremely fortunate boon for Iberia, and indeed a large share of young men did go 'to conquer' there; however, one wonders what those young men would've done had there not been an untamed continent to conquer? If say, their country didn't have ready access to the Atlantic? The consensus seems to be that the European population at large was reaching pre-Black Death levels toward the beginning of the 16th century. Prior to discovering America the Iberian states wouldn't have had a distinct advantage over their neighbours-so why didn't the 'wave' resulting from Pope Innocent in 1484 have effects elsewhere? Even proposition the the Pope's proclamation even had a great effect is questionable. Not only can I not find support in demographic resources, I highly doubt (but I'm not an expert) contraception had a statistically significant effect on the population in the first place. Overall, I'd take this field of study with a healthy dash of salt for now, and wait for it to go mainstream or published in English to see if its complete bollocks.
Heinsohn's influence on serious study? I'd wager very little to nil. Trykkefrihed.dk (where the interview is) is a highly political publication and I very much doubt he took an interview there by accident. Historians who use their research to push political agendas are never popular. Not to mention his field isn't technically history: John M. Riddle's Reply to his theory is titled "The Great Witch-Hunt and the Suppression of Birth Control: Heinsohn and Steiger's Theory from the Perspective of an Historian"
EDIT: Here is what I used to look up population data