A country is not monolithic. Your question almost answers itself if you are more specific: "Would the poorest 10% be better off?" Or "would the top 1% be better off?"
You also should ask what you try to measure. For example, GDP may rise enormously without the lower 10% profiting, as has been the case in the U.S. since the 1970s (see e.g. https://www.epi.org/publication/charting-wage-stagnation/, fig. 4). Even in the most developed country with a more or less working democratic government, the poor do not benefit from trade and economic growth at all. The idea that the poor profit more in kleptocracies or countries run by autocratic governments, which is the case in much of Africa and Asia, is not plausible.
For a case study, I recommend the documentary "Darwin's Nightmare". It shows the effects of opening the fish economy around lake Victoria for the international market on the local population. That the poorest do not profit is not a singular observation:
the postulated [by the Stolper-Samuelson theorem] narrowing wage gaps between skilled and unskilled labour have not been observed in many developing countries, particularly in Latin America and Africa. [Machiko Nissanke/Erik Thorbecke, UNU-Wider]
This is corroborated by Nina Pavcnik from Dartmouth College:
However, workers who were employed in sectors that were initially shielded by higher tariffs experienced a drop in relative wages as tariffs were eliminated. Many countries, such as Mexico and Colombia, had shielded industries that employed a high share of less educated workers. When the tariffs were eliminated, these unskilled workers were disproportionately affected by declines in industry wages. These are short-term costs of globalization, and over time you would hope that these workers would be able to move toward the exporting sectors and share in the benefits of globalization. But that is not occurring as fast as we would like because worker mobility in many of these countries is quite constrained.
Hope never dies, but I'd like to point out that what should be short-term effects in theory turn out to be long-term in practice.
The eminent Ann Harrison has written a very readable 2006 paper Globalization and Poverty for the National Bureau of Economic Research which one can succinctly summarize with the following quote (p.7):
Globalization produces both winners and losers among the poor.
She explores cross-country, cross-region and cross-sector differences and notes that because globalization affects sectors differently, labor mobility is a key factor mitigating negative impacts on the poor, together with a number of other factors. Here are some key findings:
- "Measures of export activity and
foreign investment are generally associated with poverty reduction, while removal of protection
(an ex ante measure of globalization) or import shares (an ex post measure) are frequently
associated with rising poverty." (p.8)
- "Don Davis and
Prachi Mishra argue that 'Stolper-Samuelson is dead'. They write eloquently that applying trade
theory to suggest that liberalization will raise the wages of the unskilled in unskilled-abundant
countries is 'worse than wrong—it is dangerous.'"
- "Easterly finds that increasing trade integration is associated with falling
inequality within developed countries and greater inequality within developing countries."
- "To summarize, there is no evidence in the aggregate data that trade reforms are good or bad
for the poor"
On the other hand, a David Dollar and Aart Kraay could show convincing statistical evidence that globalization is beneficial for the poor. They divide the nations into "non-globalizers" and "globalizers" and show that the globalizers experience larger economic growth in the 1980s and 1990s. Because there is no significant change in inequality, this growth translates directly into income gains for the poor.
How can these contradicting findings be reconciled?
One hint can be found in Dani Rodrik's One Economy/Many Recipes, Princeton Press 2007. In the last chapter he asks "Globalization for Whom?" and notes that India and China "house more than half of the world’s poor, and their experience is perhaps enough to dispel the collective doom elsewhere."
He continues to show that these strongest-growing and "strongest-globalizing" economies yielding the statistical evidence in "globalizers vs. non-globalizers" studies were not playing by the WTO free-trade rules but instead protected their markets and, in the case of India, actually grew already fast before opening up to more trade. That aligns somewhat with Harrison's findings above that simple liberalization is less beneficial for the poor than direct investment.
Bottom line: It is a mixed bag, there are winners and losers among the poor between countries and within countries. Opening an economy for trade hurts the poor less when accompanied by policy measures enabling labor mobility. Free trade is less beneficial than direct investments.