Men’s stress levels increase when their wives earn more than 40% of the household income, concludes a new study out of the University of Bath.

“These findings suggest that social norms about male breadwinning — and traditional conventions about men earning more than their wives — can be dangerous for men’s health,” study author Joanna Syrda, PhD, an economist at the University of Bath’s School of Management in the U.K., said in a statement.

Syrda’s study, published in the Personality and Social Psychology Bulletin, followed 6,000 American heterosexual married or cohabiting couples for 15 years, controlling for total household income. Men reported some anxiety from being the sole breadwinner, which decreased as their partners began earning and reached it’s lowest point when wives were bringing in 40% of household income. Beyond that — as women’s incomes rose to equal or surpass their male partners’ — husbands’ stress levels increased, surpassing even the distress they felt from being the sole earner.

The exception to the finding was men whose wives outearned them prior to marriage.

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For most men, this leaves a very narrow window in which they are not stressed by their wives’ earning power. Chronic stress, either from living up to the norm of being the sole breadwinner or from the subversion of it by being the secondary earner, can affect men’s physical, mental and emotional health, Syrda suggests.

Such persistent distress — characterized in surveys filled out by both husbands and wives as feeling sad, nervous, restless, hopeless, worthless, or that everything was an effort — could also affect life satisfaction, marital fidelity, divorce, and marital power dynamics, she added. Previous research has found women in high-earning, top leadership positions in the private and public sectors face higher rates of divorce due to the shift in power dynamic, The Swaddle has reported.

Syrda refers to these dynamics as “bargaining power,” which tend to favor whoever has the higher income — traditionally men. Men whose wives outearn them may feel financially vulnerable — not having been conditioned, as women have been, to accept such a dependent state — if the relationship degrades, and separation or divorce becomes a possibility. Bargaining power also matters, “if partners have a different view on what is best for their family, how much to save, what to spend their money on, and various plans and big decisions,” Syrda writes for The Conversation.

Syrda’s research might focus on a niche, but growing, phenomenon, but its findings speak to a larger dissociation within society. Wives in the study rated husbands’ stress lowest when the income was netted 50-50, while men reported their stress was actually lowest when wives were only contributing 40%.

“This too may be down to gender norms. If masculine social roles preclude the admission of vulnerability, and men are inclined to hide symptoms of stress and depression, it follows that wives’ responses about their spouses will be less accurate,” Syrda said.

Overall, men in the study were happier when their partners contributed to the household income, which could be seen as progress. But “as the number of women outearning their male partners grows, the traditional social norm of the male breadwinner may begin to adjust,” Syrda concluded.