Lead
Most parents remember the first time their child said "that's not fair." It might have been directed at a younger sibling getting a parent's attention, or at a friend who had a toy the child didn't. The phrase carries a weight that surprises — it sounds both deeply felt and somehow adult.
Is it an emotional expression, a moral complaint, or both? The psychology of jealousy and envy — how these two distinct emotions develop in children, and how children eventually find words for them — offers a more specific answer than we might expect, and it changes how the phrase "that's not fair" sounds to a parent who hears it.
Jealousy and envy have different structures
In everyday conversation, jealousy and envy are often used interchangeably. Psychology distinguishes them.
Jealousy is a three-party emotion. It arises when a valued relationship — with a parent, a friend — is perceived as threatened by a third party. The emotional center is fear of losing something already held.
Envy is a two-party emotion. It arises from a comparison in which the other person has something you don't. The emotional center is dissatisfaction that another has what you lack.
When a child says "that's not fair," which one is it? It depends on the situation. A child who says it while watching a parent hold a younger sibling is closer to jealousy. A child who says it in response to a friend's new toy is closer to envy. Both words in Japanese often collapse into the same expression — zurui (ずるい), roughly "unfair" or "no fair" — which makes it worth slowing down and listening to the context.
Jealousy is present at six months
The evidence that jealousy exists long before children have language for it comes from a carefully designed experiment by Hart and Carrington (2002) [1]. Thirty-two six-month-old infants were observed as their mothers directed attention to one of two objects: a realistic infant doll (a social object) or a book (a non-social object). Infants showed significantly more negative affect — looking away, protest behaviors, distress — when their mothers engaged with the doll than when they engaged with the book. The difference was specific to the social nature of the rival, not simply to the mother's redirected attention.
This is interpreted as a proto-jealous response: the six-month-old cannot understand the concept of jealousy, but can already respond emotionally to the threat of a social rival receiving a valued person's attention. Language and conceptual understanding arrive much later; the emotional reaction does not wait for them [1].
For sibling jealousy specifically, Volling, McElwain, and Miller (2002) studied 60 families with an infant around 16 months old and an older sibling, observing the older child's emotional reactions when parents attended to the younger one [2]. Their data showed that sibling jealousy is not a single clean emotion — it presents as a complex of sadness, anger, and anxiety, mixed in different proportions depending on the child's temperament and the family's parenting patterns. When a child says "that's not fair" as a parent picks up a baby, that single phrase may be carrying multiple emotional streams at once [2].
The gap between feeling and naming
Emotional experience consistently precedes the language for it. Children feel jealousy at six months; they can name something like it at four years at the earliest. Ridgeway, Waters, and Kuczaj (1985) studied receptive and productive vocabulary for emotion words in approximately 120 children ages 18 months through six years [3]. Basic emotion words — happy, sad, mad, scared — appear in production by 18 to 24 months. Words corresponding to envious or jealous did not appear reliably until after age four.
To label a complex emotion like envy, a child needs more than the emotional experience. They need the cognitive capacity to hold a comparison in mind: "that person has something, and I don't, and that matters." Envy and jealousy require a relatively sophisticated self-other comparison — the kind of comparison whose neural and cognitive development is itself still underway in the preschool years.
In Japanese, zurui is a usefully elastic word: it covers envy, jealousy, and unfairness simultaneously. When a child uses it, the underlying emotion is not necessarily one thing. Asking gently "Why does that feel unfair to you?" — rather than responding immediately to the surface word — opens a space where the child can articulate something more specific, and where the parent can hear what is actually going on. That exchange is itself one of the mechanisms through which children develop emotional vocabulary.
The development of fairness — "why does equal matter?"
Closely related to envy is <em>inequality aversion</em>: a psychological preference for fair, equal outcomes; people often dislike unequal distributions even when the inequality benefits themselves — the sense that unequal distributions are intrinsically objectionable, not merely personally inconvenient. Fehr, Bernhard, and Rockenbach (2008) published findings in Nature from a resource distribution experiment with 229 children ages three to eight [4]. Three- and four-year-olds largely accepted distributions that favored themselves over others. By ages seven to eight, children showed strong aversion to both self-advantaging and self-disadvantaging inequality — preferring equal distributions even when unequal ones would leave them with more.
The preference for equality, in other words, is not innate — it develops. A four-year-old who says "that's not fair" because a sibling got more strawberries is operating with a mix of personal envy ("I want what they have") and an early, partial version of fairness reasoning. The full-blown moral principle that equal distributions are right regardless of who benefits does not appear until several years later.
One additional finding from the Fehr et al. data worth noting: the equality preference was stronger toward in-group members — children in the same perceived social group [4]. "That's not fair" tends to be most intense in close relationships, with siblings and best friends. That intensity is, in a sense, also evidence of closeness. The complaint is loudest where the relationship matters most.
What parents can do
The gap between when children feel complex emotions and when they can name them is the key practical fact here.
Don't dismiss the feeling by disputing the word. Responding to "that's not fair" with "it is fair" or "stop being jealous" treats the surface word as the issue. The child's emotional experience — whichever of the overlapping emotions it actually is — remains unaddressed. What tends to work better is reflecting the feeling back with a name: "You wanted a turn with that too, and it felt bad watching them get it — was that it?" Offering a more specific label gives the child a more precise word to work with next time.
Use emotion words in conversation, including for your own feelings. Children acquire emotional vocabulary partly by hearing adults use it in context. Saying "I felt a bit envious when I saw that" or "I was jealous of the time they got with Grandma" — not performatively, but when it is true — provides models that children absorb and begin to apply to their own experiences.
Recognize the relational signal in the complaint. A child who protests loudly that a sibling is getting more time, more strawberries, or more praise from a grandparent is, in part, expressing something about how much that relationship matters to them. That does not mean the demand should always be met, but it is worth hearing what is underneath it before responding.
Keep a record of the language that emerges. Moments when a child first uses a specific emotion word — jealous, envious, left out — are developmentally significant. Logging these in a parenting record gives parents a way to trace the arc of emotional vocabulary development over months, which can be as revealing as tracking physical milestones.
Summary
Jealousy and envy are structurally distinct emotions. Jealousy is present as early as six months [1], but language for it does not arrive reliably until age four or later [3]. The phrase "that's not fair" typically reflects a complex of emotions — envy, jealousy, and early fairness reasoning — not a single clearly formulated complaint [2,4].
Children who say it are not being manipulative. They are using the most precise emotional language they currently have for something they are genuinely experiencing. Meeting that experience with a more specific name, rather than a corrective response, is one of the most effective ways to help children develop the emotional vocabulary that, over time, makes complex feelings more manageable to live with.
Feelings are considerably easier to handle once they can be spoken.
References
- Hart SL, Carrington HA. Jealousy in 6-month-old infants. Infancy. 2002;3(3):395–402. doi:10.1207/S15327078IN0303_6. PMID: 33451216.
- Volling BL, McElwain NL, Miller AL. Emotion regulation in context: the jealousy complex between young siblings and its relations with child and family characteristics. Child Dev. 2002;73(2):581–600. doi:10.1111/1467-8624.00425. PMID: 11949910.
- Ridgeway D, Waters E, Kuczaj SA. Acquisition of emotion-descriptive language: receptive and productive vocabulary norms for ages 18 months to 6 years. Dev Psychol. 1985;21(5):901–908. doi:10.1037/0012-1649.21.5.901.
- Fehr E, Bernhard H, Rockenbach B. Egalitarianism in young children. Nature. 2008;454(7208):1079–1083. doi:10.1038/nature07155. PMID: 18756249.
- Masciuch S, Kienapple K. The emergence of jealousy in children 4 months to 7 years of age. J Soc Pers Relat. 1993;10(3):421–435. doi:10.1177/0265407593103008.