International study highlights the costs of allowing the wealthy to opt out of collective solutions to global problems such as climate change
Monday 23 Mar 2026
Allowing wealthy countries, communities, or individuals to fund their own solutions to collective action problems such as climate change is inefficient, increases inequality and leaves vulnerable communities unprotected – but the 'private solution trap' can be averted with specific policy interventions, according to a new global study published in the Proceedings of the National Academy of Sciences (PNAS) based on an experimental game played by 7,500 students in 34 countries.
The research was conducted by the University of Warwick's Collective Decision-Making and Culture Lab (CDMCL), an international and interdisciplinary group of researchers, in 34 countries under the leadership of Dr Eugene Malthouse (University of Nottingham), Professor Thomas Hills (University of Warwick and the CAGE Research Centre) and Professor Daniel Sgroi (University of Warwick).
Global collective action problems such as addressing climate change and tackling pandemics require international co-operation if they are to be solved. But in an unequal world, wealthy individuals and nations can opt out of collaborative 'public' solutions and fund their own private solutions – such as building flood defences and excessively stockpiling vaccines.
These private solutions form a trap because, according to the study, they can divert resources away from more efficient shared solutions, exacerbate existing inequalities, and leave more vulnerable individuals and communities unprotected.
In order to understand more fully the choices people make between public and private solutions, the researchers created a game in which groups of four students – two "rich" and two "poor" – were given high or low budgets that they could invest in a public solution (benefiting all group members) or a private solution (benefiting only themselves) across ten rounds. Achieving either solution meant they avoided losing their remaining budget.
The study finds:-
- Only 15 of the 1,151 groups opted wholly for the public solution; while just 11 opted wholly for private solutions. The majority of groups were split between the two options.
- Across all 34 participating countries, the "rich" consistently invested more in private solutions than the "poor": 62 per cent of rich players adopted the private solution compared to 32 per cent of poor players
- Poor players contributed a greater proportion of their funds towards the public solution than their rich counterparts – 40 per cent compared to 31 per cent.
- As a result of these patterns of behaviour, wealth inequality had increased in every country by the end of the game.
- The odds of poor players losing everything were estimated to be more than five times higher than they were for rich players
- Players in countries which value harmony above hierarchy were more likely to invest in the public solution.
Dr Malthouse said:
"Our results highlight the potential consequences of the private solution trap whereby the existence and widespread adoption of private solutions undermine the provision of public solutions, while also increasing wealth inequality and leaving less-wealthy individuals and nations unprotected against collective risks."
Professor Sgroi said:
"We found two universal pathways which helped groups achieve the collective 'public' solution. These applied across almost all countries and cultures.
"The first is early intervention – the larger the sum invested in the public solution in round one, the more likely the group is to adopt the public solution. The second is conditional co-operation – groups in which players reciprocated the contributions of their fellow group members were more likely to adopt the public solution.
"We can see examples of this in the struggle against climate change where it can often feel like richer countries are hanging back and waiting for other nations to take the lead. If a smaller country takes the leap and commits its more limited resources to a collective solution, the developed world often falls in behind."
"It's especially interesting that the UK is something of an outlier in our results. Our results show that in the UK if someone starts early by co-operating that makes a big difference to the odds of the whole group adopting the public solution.
"This suggests that policies that try to kick-start cooperative behaviour will be especially effective for the UK compared to some of the other countries - it does seem to really inspire people to change their behaviour."
Co-author Professor Thomas Hills of the University of Warwick, added:
"Private solutions are a great form of insurance for those who can afford them. But their mere existence destabilises public goods – for example, people hiring private fire trucks for their burning homes in LA made those goods more expensive for everyone and completely out of reach for the poor.
"The data shows clearly this is a problem that exists above culture. Some countries do better than others, but they all fall prey to the private solution trap. Perhaps the key optimistic point is that we can do better, by investing in public solutions more often and more quickly. Then private solutions lose their appeal."
- E. Malthouse, C. Pilgrim, D. Sgroi, M. Accerenzi, A. Alfonso, R.U. Ashraf, M. Baard, S. Banerjee, A. Belianin, S. Bhattacharjee, M. Bhattacharya, P. Brañas-Garza, J. Cárdenas, M. Carriquiry, S. Choi, G. Clochard, E.E. Denzon, B. Dessoulavy-Sliwinski, G. Dini, L. Dong, A. Ertl, F. Exadaktylos, E. Filiz-Ozbay, S.L. Flecke, F. Galeotti, T. Garcia-Muñoz, N. Hanaki, G. Hollard, D. Horn, L. Huang, D. İriş, H.J. Kiss, J. Koch, J. Kovářík, O.K.B. Kwarteng, A. Lange, M. Leites, T.H. Leung, W. Lim, M. Morren, L. Nockur, C.Y. Okyere, M. Oudah, A.I. Ozkes, L. Page, J. Park, S. Pfattheicher, A. Proestakis, C. Ramos, M. Ramos-Sosa, M.S. Ashraf, M.R. Sanjaya, R. Schwaiger, O. Sene, F. Song, S. Spycher, R. Staněk, N. Tanchingco, A. Tavoni, V. te Velde, M.J. Vázquez-De Francisco, M. Visser, J.T. Wang, W. Wang, W. Weng, K. Werner, A. Wijayanti, R. Winkler, J. Wooders, L. Ying, W. Zhen, & T. Hills, The private solution trap in collective action problems across 34 nations, Proc. Natl. Acad. Sci. U.S.A. 123 (12) e2504632123, https://doi.org/10.1073/pnas.2504632123 (2026).
- The study was jointly funded by the University of Warwick's Global Research Priorities, the European Research Council, the Spanish Ministry of Science and Innovation, and universities in each participating country.
- While the primary focus of the study was climate change, the findings apply to any collective problem in which private and public solutions are available – from education, to healthcare, to security.
