On average, 5 percent of the populations of Britain, Ireland, and Norway emigrated every decade between 1850 and 1910, which increased to 14 percent of the Irish population emigrating between 1890 and 1910. By the turn of the century, Italy, Portugal, and Spain recorded similar emigration levels... The Swedish population fell by 44 percent in the twenty-year period from 1871-1890.
Massive immigration dramatically impacted the economies of countries in the New World, which had relatively small populations. Between 1880 and 1910, Argentina received the equivalent of 20 percent of its population per decade; the United States between 5 and 10 percent per decade; and Canada between 5 and 15 percent per decade. Immigration in this age of mass migration accounted for around 50 percent of Argentina's population increase, and about a 30 percent increase for the United States and Australia.
From here quoting this book. A comment to the linked post references regarding immigration politics of the time: Goldin, C, “The Political Economy of Immigration Restriction in the United States, 1890 to 1921.” In The Regulated Economy: A Historical Approach to Political Economy, ed. C. Goldin and G.D. Libecap, 223-258. Chicago: University of Chicago Press.
What was so awful in Europe that everyone wanted to get the heck out of there? Stories of misery and poverty driving people to leave Ireland after the potato famine and Southern Italy are familiar from immigrant narratives. But, I'm not familiar with any similar narrative for late 19th century Sweden. Yet, surely you wouldn't see a 44% exodus of Swedes from their homeland in twenty years if everything was fine and dandy in Sweden. Sure, the northern Midwest is full of Swedish Lutherans, but it had never occured to me that they would have emigrated in such large numbers relative to the population of Sweden. I'd always imagined that event as something more like a small subculture young people who set out to seek their fortune because the grass was reputed to be greener across the Atlantic, answering a call that most people ignored.
My own family history recounts the looming war of 1848 as a push factor in the migration of my patrilineal ancestor to the United States from Germany.
But, the story of my maternal Swede-Finn ancestors' migratration from Finland to the Upper Pennisula of Michigan a few decades later as teenagers isn't accompanied by a narrative that would suggest any push factor that dramatic. (Finland was a Grand Dutchy within Russia from 1809 to 1917, having been acquired in resolution of a war between tsarist Russia and Sweden, and was previously a part of the Swedish empire; Finnish emigration peaked around 1900, a bit later than than Swedish neighbors, perhaps because it industrialized a little later.)
According to Wikipedia:
Sweden—much like Japan at the same time—transformed from a stagnant rural society to a vibrant industrial society between the 1860s and 1910. The agricultural economy shifted gradually from communal village to a more efficient private farm-based agriculture. There was less need for manual labor on the farm so many went to the cities; and about 1 million Swedes emigrated to the United States between 1850 and 1890. Many returned and brought word of the higher productivity of American industry, thus stimulating faster modernization.
The late 19th century saw the emergence of an opposition press, the abolition of guild monopolies on craftsmen, and the reform of taxation. Two years of military service was made compulsory for young men, though there was no warfare.
The steady decline of death rates in Sweden began about 1810. For men and women of working age the death rate trend diverged, however, leading to increased excess male mortality during the first half of the century. There were very high rates of infant and child mortality before 1800, Among infants and children between the ages of one and four smallpox peaked as a cause of death in the 1770s-1780s and declined afterward. Mortality also peaked . . . due to other air-, food-, and waterborne diseases, but these declined as well during the early 19th century. The decline of several diseases during this time created a more favorable environment that increased children's resistance to disease and dramatically lowered child mortality.
Thus, rather than war or disease, the push factor appears to have been an increased supply of young people as mortality fell with modernity accompanied by a declining job market due to the mechanization of the economy. There are parallels to the enclosure movement in Britain as well as to the mass displacement of tenant-farmers in Japan referenced in Wikipedia. The coincidence of the imposition of compulsory military service and the mass migration is also notable.
This example also suggests give the very large percentages involved relative to the populations involved on both ends of the journey that there was a strong potential for population genetic selection in the New World for any trait that would predispose someone to make a migration and in the Old World for any trait that would predispose someone to not making a migration.