The Biology Behind Human Relationships

Relationship advice usually focuses on conscious actions: Don’t go to bed angry. Develop good communication skills. Go on a weekly date. These are a few popular examples. While this advice is not bad, it ignores some basic information from your high school biology class. What if a large part of relationship functioning is completely unconscious? Researchers at BYU and around the world suspect that is exactly the case. Building on international research, three professors from BYU’s Marriage and Family Therapy program have a theory that ancient biological processes are driving our feelings and behaviors in relationships today.1 In this three-part series we will explore the work of these researchers and what it means for relationship connections, healing from trauma, and promoting physiological health and awareness.

The autonomic nervous system (ANS) is the body’s primary regulatory system. The ANS is made up of two systems: the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS). These two systems balance your body between action and rest. As you go about your day, the brain takes in information from the senses and processes those perceptions into one coherent experience. Part of this constant intake of information is to decide if the information is important and relates to survival or protection. If the brain interprets something as a threat, it signals the SNS to become active. The SNS is responsible for the “fight or flight/freeze” response. An activated SNS is characterized by the secretion of stress hormones, which increase heart rate, breathing, and blood pressure.2 Your body starts reacting before you are even conscious of your pounding heart1.

The PNS balances the SNS, acting like brakes on your brain’s protective reactions. The PNS calms you down and puts you in a state of rest and digestion, which facilitates interpersonal connection. When the PNS is in control, you are able to connect with others, which is impossible in a state of high SNS arousal.

These systems of reaction and rest are ancient. The SNS functioned in the unpredictable, dangerous environment of your prehistoric ancestors, and functions today when something is perceived as a threat. Because of the success the SNS has had in keeping us alive, it performs in basically the same way it always has. It may seem primitive, but it gets the job done.

The fight or flight response has a relationship equivalent. When confronted with an argument or other potentially stressful situation, your conscious thoughts know that you are not facing off with a tiger, however, your SNS is responding as if you are. The fight or flight response in relationships has been called the demand-withdraw cycle. In the demand-withdraw cycle, one partner is the demander, seeking change, discussion, or resolution of an issue. The other partner is the withdrawer, seeking to end or avoid the demander’s requests3.

A powerful way to combat the demand-withdraw cycle is to seek to engage the PNS. Stress reduction, exercise, and basic sleep and nutrition are some things that engage the PNS. Turns out “don’t go to bed angry” can be really bad advice. An angry fight at bedtime likely involves a lot of SNS response – a good night’s sleep may be just what your relationship needs.

The next article will focus on trauma (big or small) and how addressing the traumas of the past is necessary for keeping the SNS and PNS in a healthy balance.


  1. Johnson, Bradford, & Miller
  2. Van der Kolk, K. (2014).
  3. Papp, Kouros, & Cummings, 2009
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Written by Kirsten Crane Cadden, with Lee Johnson, Ph.D., LMFT & Angela B. Bradford, Ph.D.

Kirsten Cadden is currently a senior at BYU majoring in family studies. As an undergraduate she has worked as a research assistant and a professor's assistant. After graduation this year, Kirsten plans to pursue a master's degree in clinical social work. Kirsten is a big fan of therapy, mindfulness, and self-care. Contributors: Lee Johnson, an associate professor in Marriage and Family Therapy at BYU, holds a bachelor’s degree from BYU in family science, a master’s degree in marriage and family therapy from Utah State University, and a Ph.D. from Kansas State University. He has worked at Friends University, University of Georgia, and now BYU. His research interests focus on examining the relationship between physical exercise, improved sleep, and reduced stress on marital and family therapy outcomes; emotional regulation process in clinical couples and families; and the therapy alliance. Angela Bradford, an assistant professor in Marriage and Family Therapy at BYU, earned her bachelor’s degree in family science from BYU, and her master’s degree in marriage and family therapy and Ph.D. in human development and family studies from Auburn University. She has worked as a university instructor, a graduate research manager, and in private practice as a therapist. Her research with Lee Johnson, and Rick Miller, is the basis for this article.