Falling in love is one of the essential human experiences. It has been romanticized for centuries, and is essential for human survival, but little is known about the scientific process behind it.

In a new study, researchers evaluated the psychological process and examined the physiological response to falling in love.

The early stage of romantic love is usually associated with “intense preoccupations and worries regarding the partner and the relationship, obsessive-like anticipation, focus on minute non-verbal signals, and fears of rejection,” the researchers from Bar-Ilan University noted. However, romantic relationships also require “sufficient calm” in order to create a trusting approach with one’s partner, which is known as “immobility without fear.”

Specifically, cortisol, a steroid hormone excreted usually in response to stress, has been related to “psychological, physiological, and physical health.” Cortisol levels are associated with behavior between romantic partners; partners with higher levels of cortisol have demonstrated greater animosity during conflict interactions.

Researchers in this study published in Adaptive Human Behavior and Physiology argue that reduced cortisol production is necessary for this state of “immobility with fear,” which is crucial to the beginning of a romantic relationship.

To test this, researchers “measured associations between daily cortisol and CAR and the couple’s observed social reciprocity and joint partnership during naturalistic interactions.” Using this theory of cortisol, researchers utilized one hundred and 55 young adults which were split into two groups — a new couples group, who began their relationships on average 2.4 months prior to the study, and a singles group, which included 35 young adults who were not in any romantic relationship and had not been so for at least 3 months prior to the study. The singles group was comprised of 21 women and 14 men. The participants collected saliva when they woke up, half an hour after waking up, and just before bedtime on two consecutive weekdays.

Couples also were asked to “arrive at the lab for a videotaped interaction that included two paradigms; positive and support giving.” In the positive interaction, couples discussed a shared positive experience; the support giving interaction had couples describe to each other situations that had caused them stress but weren’t related to the relationship.

Researchers found that couples in relationships produced less cortisol than individuals not in relationships, like they had predicted. Romantic partners who displayed “greater social reciprocity and goal-directed partnership, including the expression of positive affect, matched dyadic states, visual attention to partner, consistent and predictable style, and focus on listening to the partner and jointly accomplishing the task at hand” during the interaction sessions at the lab were also shown to have lower cortisol production than those couples who did not.

One important limitation in the study was that the singles group was comprised of mostly women; the results could show some gender bias. Also, the amount of time couples spent together during the study was not measured or regulated. Researchers noted that further research is necessary to study the lack of a stress response when falling in love.