In 1992, American anthropologist Helen Fisher scanned the brains of people who had just fallen in love, and found something uncomfortable: the regions active during love were almost identical to those active during cocaine addiction. The same neural circuits. The same dopamine euphoria. This awkward discovery overturned our understanding of what “choosing a partner” actually means — and turned out to be central to understanding the biology of co-parenting.
Love is not a single feeling. It is three sequential biochemical states, each with its own hormonal basis, its own evolutionary purpose and its own time limit.
The first phase is the most straightforward. Testosterone in both sexes intensifies sexual interest: it is not a “male hormone” but a universal initiator of desire. In women, testosterone peaks around ovulation — and it is precisely then that the attractiveness of potential partners is evaluated most keenly.
Oestrogens add sensitivity to social signals: facial expressions, tone of voice, scent. Research by Norbert Swift (University of Vienna, 2009) showed that women in the fertile phase of their cycle recognise emotions from micro-expressions significantly more accurately than at other times. Hormonal state literally sharpens social perception at the moment when it is evolutionarily most useful.
This is Fisher’s territory. Romantic passion is driven by the dopamine reward system: the release of dopamine in the ventral tegmental area creates the “I think about you every minute” state. This is not a metaphor — it is a measurable neural process.
Simultaneously, serotonin drops. This matters: the lowering of serotonin explains the intrusive thoughts about the partner and the anxiety of separation. Italian psychiatrist Donatella Marazziti, in 1999, compared serotonin levels in people in the throes of falling in love with those in patients with obsessive-compulsive disorder. They were statistically indistinguishable.
The duration of this phase: a few months to two or three years. Then the dopamine response to the partner inevitably diminishes through habituation. Evolution programmed a limited duration of “passion” so that the brain does not spend all its computational power on one person indefinitely.
Being in love is not a character trait. It is a temporary neurochemical state. This is important to understand before making long-term decisions about parenthood.
The third phase is the most important for co-parenting. Oxytocin is released during physical contact, feeding, and shared caregiving. Vasopressin contributes to territorial loyalty — the tendency to “guard” one’s own.
The classic experiment: prairie voles (Microtus ochrogaster) are among the very few rodents that are monogamous in the wild. When their oxytocin receptors are blocked, they lose monogamous behaviour. When oxytocin is administered to montane voles (normally not monogamous), they begin to show signs of attachment. One hormone, one receptor — fundamentally different social behaviour.
In humans: long-term couples show synchronisation of oxytocin and cortisol levels — a physiological “tuning” to each other. Couples with stable attachment show lower systemic inflammation and slower telomere shortening. This is not romance — it is medical data.
In most female primates, the fertile period is visually marked: genital skin reddens and swells. The signal is read clearly — and triggers male competition. In humans, this does not happen. Why?
The most compelling hypothesis belongs to anthropologist Sarah Blaffer Hrdy (1979): concealed ovulation keeps men close continuously. If a man cannot tell when the fertile moment arrives, it is in his evolutionary interest to remain present at all times. This increases the probability of paternity — and incentivises investment in the child.
A parallel observation: in species that openly display ovulation, competition among males for the fertile moment is far more intense. In gorillas — strict hierarchy, one silverback monopolises reproduction. In chimpanzees — complex competitive coalitions. In humans — stable pair bonds with both parents involved in child-rearing. Concealed ovulation made a different social model possible.
Additional confirmation: research shows that women’s behaviour shifts subtly across the menstrual cycle — voice pitch rises slightly in the fertile phase, clothing choices shift. Men respond to these signals without consciously perceiving them. This is a neuroendocrine dialogue conducted entirely below the level of awareness.
Concealed ovulation is an evolutionary invention that transformed sexual contact from a momentary event into the foundation of long-term partnership.
A separate story about how we choose partners — and how the immune system participates. In 1995, Swiss biologist Claus Wedekind conducted an experiment: men wore cotton T-shirts for two days without using deodorant. Women smelled the shirts and rated the attractiveness of each scent.
The result was statistically clear: women consistently preferred the scent of men whose MHC profile (major histocompatibility complex) differed most from their own. The MHC is the part of the immune system that recognises “self” from “non-self.” A diverse MHC between partners means a broader immune repertoire in offspring.
Wedekind included a control group: women taking oral contraceptives showed inverted preferences — they favoured similar MHC profiles. Hormonal state literally altered the immune “taste.” Some researchers believe this explains a portion of the cases where couples report reduced mutual attraction after stopping contraception — the woman is again “hemaring” her partner’s scent differently.
When a pair — romantic or parenting — is stable, physiological synchronisation occurs. This is not a metaphor.
Neuroscientist Ruth Feldman at the Hebrew University studied mothers and fathers with newborns. Her finding: in both parents, contact with the child activates the same neural circuits — regardless of biological kinship. In fathers who were the primary caregivers, the brain activity pattern was virtually identical to that of mothers. Oxytocin was released in both upon physical contact with the child.
What this means for co-parenting: biological attachment to a child is not the mother’s monopoly. It is formed through practice — through repeated contact, shared caregiving, shared rituals. A co-parent who is actively involved from the first days becomes neurobiologically “tuned” in the same way as a biological parent.
For children this is critical. A 2019 meta-analysis (Cabrera et al., Psychological Bulletin) on data from 45,000 families found that active involvement of both parents in the early years is significantly associated with better outcomes in emotional regulation, stress resilience and social skills at ages 7 to 12. Whether the parent is biological or not is secondary. Presence and contact are primary.
Several genetic polymorphisms have measurable effects on how people form attachments.
OXTR (oxytocin receptor gene): the variant rs53576 is linked to differences in empathy, capacity for social recognition and sensitivity to social support. Carriers of the AA genotype show modestly reduced empathic accuracy and more often report difficulties with closeness.
AVPR1A (vasopressin receptor gene): the RS3 repeat sequence in the gene’s promoter is associated with differences in the tendency toward monogamous relationships. A 2008 study (Walum et al., PNAS) on 552 Swedish couples found that men carrying two copies of the long RS3 allele were significantly more likely to report difficulties in close relationships.
DRD4 (dopamine D4 receptor): the long variant (7R) is associated with novelty-seeking and lower tendency toward stable pair bonds. The same variant is linked to ADHD. This is not a “gene for infidelity” — it is a marker of a particular neurobiological profile.
An important caveat: each of these genes explains only a small fraction of the variation in actual behaviour. This is not determinism — these are biological tendencies that environment, upbringing and conscious choices can substantially modify.
The other side of attachment: when it breaks — it is not only emotionally painful. It is physiologically measurable.
Chronic conflict in a relationship elevates cortisol in both partners. Long-term elevated cortisol is one of the strongest known drivers of accelerated telomere shortening. People in conflictual marriages have shorter telomeres in immune cells than people in stable relationships or people living alone without chronic stress. This is from a 2018 meta-analysis (Liu et al.) across 34,000 participants.
For those thinking about having a child: this means that the quality of the relationship between parents affects not only a child’s psychology, but the biological environment in which they develop. A conflictual environment is a background of chronic stress that alters epigenetics. A peaceful shared model is a different biochemistry — literally.
Love is not just a feeling. It is three sequential neurochemical states with different hormones, different time frames and different evolutionary functions. Concealed ovulation is an evolutionary invention that made long-term pair bonds possible. The immune system participates in partner selection through the MHC profile — via scent. Attachment forms through practice — and is available to anyone who shows up.
All of this has direct relevance to conscious co-parenting: biology is neither an obstacle nor a guarantee. It is the material that deliberate choices work with.
The chemistry of falling in love creates the illusion that the partner is perfect. The biology of attachment creates the conditions in which two people can raise a child. These are different processes — and it is fortunate that both exist.
Module 1 (Matching & Co-Parenting) contains a structured list of questions for a first meeting with a potential co-parent — including psychological aspects of attachment and stress response. The Immunity and Pregnancy article in the Learn section explains how MHC compatibility affects offspring health.
a neuropeptide of the hypothalamus, released during physical contact, feeding and shared caregiving. The key hormone of attachment and trust formation.
a group of immune recognition genes. Maximum MHC difference between partners broadens the offspring’s immune range. Recognised through scent.
the oxytocin receptor gene. The variant rs53576 is linked to differences in empathy and sensitivity to social signals.
a neural network that generates motivation and “euphoria.” Activated during romantic passion in the same way as by stimulant substances.
the physiological alignment of oxytocin and cortisol levels in long-term partners. Associated with more effective joint caregiving and reduced stress reactivity.