You have to compare levels of motivation.

From Polyamory Offers A “Unique Opportunity” To Enjoy Prolonged Passion And Closeness In Romantic Relationships by Emma Young. One writer whom I quite respect for his evidence-based approach and his rationalism is a keen polyamorist but otherwise I have no particular connection to this debate.

Marriage cultural norms patterns are clearly associated with certain estimable outcomes such as productivity, longevity, well-being, but their specific causal mechanisms are deeply speculated but little proven. Monogamous marriage at a later age (mid-twenties), to out-members (i.e. non-cousin marriage), in a nuclear family context is associated with all the early members of the Age of Enlightenment countries and the industrial revolution (i.e. those behind the hajnal line).

This particular research is wretchedly speculative. Small sample size, self-selected sample, internet based, no apparent control of confounding variables (particularly geography), and, most critically, an apples-and-oranges comparison.

Rhonda Balzarini at York University, Canada and colleagues conducted the first empirical test of differences between eroticism and nurturance among participants who were in either a monogamous or a polyamorous relationship. Someone in a polyamorous relationship typically has a primary partner (they usually live with this person who is often their spouse and the co-parent of their children if they have any) and also, with the consent of that person, a secondary romantic partner. Relationships with secondary partners tend to last for at least a few years, allowing for some nurturance, as well as sex.

The researchers recruited their polyamorous participants – more than one thousand of them – from Facebook and Reddit groups dedicated to polyamory discussions. These individuals had been with their primary partner for an average of seven years, and with their secondary partner for two. They completed questionnaires that asked about levels of nurturance (to what extent they felt a strong sense of security, love, warmth, etc) and eroticism (including their felt levels of desire and lust, and sexual excitement) in each relationship, how close they felt to each partner, and also their levels of sexual satisfaction. The team also collected similar data on over two thousand monogamous people, who had been in an exclusive relationship for an average of 17 years.

It does, however, highlight a particular surveying issue which is recognized but rarely discussed. Participant asymmetric motivation.

I am making up the numbers in order illustrate the likely issue.

Among all stable relationships, I'll assume that less than 5% are polyamorous. I suspect it is probably less than 1% using their definitions, but let's stipulate 5%.

They recruit 1,000 participants in polyamorous relationships. They recruit 2,000 representing traditional monogamous relationships.

Let's also hold aside that they are all self-selected, there are few or no observational controls, there appears to be no controls around standard confounding factors such as income, educational attainment, geographical location (both by nationality and by urban/rural type issues), etc. I hypothesize that the polyamorous community to have higher education credentials, more urban, higher income, less religiously observant, perhaps more ethnically homogenous, more white collar services class, fewer children, etc. than the traditional monogamists. That's not an inconsiderable number of confounding factors which ought to be controlled.

However, there is another which is rarely focused on but which this study highlights. Motivation.

Given, at 5%, that polyamory is socially deviant from norms (the other 95%), anybody pursuing polyamory must almost necessarily be highly motivated. There are all sorts of social and legal norms which raise the regulatory barriers, economic opportunity costs, and social costs to polyamory. The commitment to polyamory must exceed the barriers and costs.

Monogamous relationships, well within the social norm, have far fewer barriers and costs. It is easier to go along with the norm.

Therefore when you compare two such populations you are effectively comparing a highly motivated population to a non-motivated population. The unlikeness between the two populations in motivational inspiration will necessarily generate its own patterns of variance which have nothing to do with the differences between monogamy and polyandry per se.

How would you control for the motivational variable? How do you find highly motivated monogamists?

However you do that, you are then likely to find that it introduces yet another uncontrolled variable. In other words, I suspect that you will find that a population of highly motivated monogamists are also highly religious compared to polyamorists.

But I also suspect that if you compare highly motivated polyamorists with highly motivated monogamists, their life satisfaction, nurturance, eroticism, etc. will also be much more similar.

It is an imperfect analogy, but perhaps illuminating. Would we expect similar levels of pride of service and life satisfaction between military volunteers and military draftees. I suspect not and the differentiating factor would be due to differences in degree of motivation.

By not controlling for motivation, beyond all the other things they don't control for, the researchers undermine their own observations.

You have to wonder why this research was undertaken in the first place. Their is so little thought put into the effort and so many glaring shortfalls in methodology, that it tells you nothing at all. Except that, perhaps, the researchers are interested in polyamoury and/or are desperate to make it a topic of conversation.