It’s well known that people who are stressed, depressed, or just plain unhappy, are more likely to develop heart and mental health issues later on in life. But according to a new study, having these personality traits may also affect fertility in both men and women.

Vegard Skirbekk, from the International Institute for Applied Systems Analysis (IIASA), and his team of Norwegian researchers examined the relationship between fertility and personality traits using birth registries and surveys of Norwegian men and women born between 1927 and 1968. They found that men with neurotic tendencies — characterized by anxiety, moodiness, envy, and jealousy — were less likely to have children, especially after 1957, compared with men who were more extroverted and open. Meanwhile, women who tended to be conscientious were also less likely to have children — according to the study, this trend was evident with all generations.

These findings are important at a time when fertility rates are dropping across Europe. Population changes factor into future projections of change in areas such as sustainability, climate, energy, and food security.

One possible explanation for these findings, Skirbekk says, could be that women and men in Norway tend to test each other more than normal before deciding to commit to raising children. Because of this, many of them are also getting married later on. “Norway is a leader country in terms of family dynamics,” Skirbekk said in a statement. “Many trends that have been observed first in Norway — increasing cohabitation, divorce rates, and later marriage, for example — have then been observed later in many other parts of the world. Of course, it remains to be seen if this phenomenon will also spread.”

It seems these rates have already been dropping across the globe. According to The Nation, some areas — such as the Middle East and sub-Saharan Africa — still have high fertility rates, but almost 60 percent of the world’s population lives in countries with below-replacement rates. A recent report by the European statistical agency, Eurostat, also found that fertility rates have declined overwhelmingly since the 1970s, particularly in Spain and Italy, according to Financial Times.    

“The recent economic crisis has left an imprint on European period fertility patterns. For some countries, it put a halt to the positive fertility trend that had started to develop. In other countries, we see significant declines in fertility,” wrote the authors of a related report published in Demographic Research.

Source: Skirbekk V, Blekesaune M. Personality Traits Increasingly Important for Male Fertility: Evidence from Norway. European Journal of Personality. 2013.