Every parent has a favorite—even if they'd never admit it.

According to new research, the child who should worry isn’t necessarily the one getting less attention. It’s both of them.

A new study following 632 pairs of identical and fraternal twins found that even small differences in how parents discipline siblings were consistently linked to mental health symptoms. In a finding that may surprise many parents, being the favored child offered no protection.

The Less-Favored and Favored Child

The new study, published in Development and Psychopathology, found that differences in discipline—not just overt favoritism—were enough to raise a child’s risk of anxiety, depression, and ADHD.

To understand why parents treat siblings differently and how those differences relate to children’s mental health, researchers studied 632 pairs of twins around 7.6 years old on average. Because identical twins share nearly all their DNA and grow up in the same home, differences between them can help researchers better isolate environmental influences—such as differences in parental treatment.

Researchers assessed parenting behaviors using home observations and questionnaires, measuring differences in discipline, intrusiveness, warmth, and favoritism. Favoritism was measured through reports from both parents and children on which sibling was perceived to receive more favorable treatment.

Children’s mental health was evaluated using standard assessments of ADHD, internalizing behaviors such as anxiety and depression, and externalizing behaviors such as aggression and rule-breaking.

Parental discipline emerged as a key predictor. When twins were disciplined differently, children showed higher levels of ADHD symptoms, as well as internalizing behaviors such as anxiety and depression, and externalizing behaviors, such as aggression, defiance, and rule-breaking.

Even small differences in how siblings were treated were linked to children’s mental health. Modest differences in discipline, intrusiveness, or favoritism were associated with higher levels of anxiety, behavioral problems, and ADHD symptoms, particularly in the child receiving harsher or less favorable treatment.

Perhaps the study’s most counterintuitive finding is what happened to the favored child.

Being favored did not necessarily protect the child. In some cases, favored children showed more externalizing behaviors, which may reflect a sense of entitlement that can develop when a child consistently receives preferential treatment.

“It turns out that siblings were very sensitive to differences in parental discipline,” Kathryn Lemery-Chalfant, a professor of psychology at Arizona State University and co-author of the study, told The Epoch Times.

Differences in negative parenting—particularly discipline and intrusiveness—were more consistently tied to symptoms than differences in positive parenting, such as warmth or affection.

Mothers and Fathers Leave Different Marks

One of the study’s more nuanced findings is that discipline from each parent was linked to different outcomes in children, suggesting that mothers and fathers shape children’s development in distinct ways.

Discipline by a mother was more strongly linked to internalizing symptoms, such as anxiety and depression. The authors suggest this may be because mothers typically spend more time with their children and handle everyday discipline matters. Children who were disciplined more by their mothers—especially the less-favored child—also tended to show higher levels of ADHD.

Fathers’ discipline showed a different pattern, specifically linked to externalizing behaviors. Children may respond to fathers’ harsher or more restrictive discipline with visible acts of rebellion—for example, by breaking rules to protest perceived unfairness.

Favoritism also played out differently for each parent. Children favored by their mothers tended to show higher levels of externalizing behaviors. In contrast, when fathers showed more favoritism toward one sibling, children were more likely to show symptoms of anxiety, depression, and ADHD.

“Father affection might be particularly important for children because it shows that fathers are invested in them,” Lemery-Chalfant said, suggesting that unequal affection from fathers may carry particular emotional weight for children.

Together, the findings highlight the importance of both parents showing consistent care and attention, not just one.

Parenting Is a Two-Way Street

However, parenting is a two-way street.

Children’s traits may also shape how they are treated just as much as parenting shapes children. To observe these dynamics in real time, researchers asked parents and children to play a novel card game together during home visits. The game was designed to create opportunities for frustration, impulsive behavior, and parent-child interaction. During these sessions, researchers observed that intrusive or hostile parenting often occurred in response to the child’s inattentive or impulsive behavior, not the other way around.

“The parents were not causing children’s negative behavior, they [the parents] were just responding to it in a negative way,” Lemery-Chalfant said.

Repeated cycles of difficult child behavior and negative parental responses can compound over time. Repeated challenging interactions can wear down parents over time, sometimes leading to fatigue, self-resentment, or feelings of inadequacy, according to Sheri Langston, a therapist and director at Rocky Mountain Therapy Group, who was not involved in the study. These factors can lead to a child with more challenging behavior being treated differently. “Even though it may only be at times, it can become a core memory for the child as an adult,” she told The Epoch Times.

The environment itself may affect both children and parents. Broader family factors—such as stress, emotional exhaustion, or inconsistency—can contribute to both elevated ADHD symptoms and differences in parenting. In some cases, parents themselves may have ADHD-related traits, such as impulsivity, which could contribute to more reactive or intrusive behavior by the children.

What to Do

No parent treats every child identically, and according to researchers, that’s not necessarily the goal. Not all unequal treatment is harmful; sometimes one child genuinely needs more support, and siblings are often able to understand this when it’s explained clearly.

The more important target, according to Lemery-Chalfant, is consistency, particularly around discipline. The best strategy is to adopt an authoritative style of parenting, which includes both parental warmth and control. “Children need to know that they are loved, but they also need parents to provide structure and routines, so children know what to expect across the day.”

She also suggests the following practical starting points:

• Have a plan before problems happen. Knowing how you’ll respond when a child breaks rules helps you stay calm and consistent.


• Use outcomes that follow directly from the behavior as natural consequences. If a child disrupts an activity, such as a game, they can be removed while play continues, reinforcing behavior without escalating emotions.


• Set clear rules with consistent consequences. Applying household rules consistently across children can reduce unequal treatment.

Parenting approaches also vary by age, Lemery-Chalfant noted. Young children need immediate, simple consequences—such as brief time-outs. In middle childhood, children are especially sensitive to fairness and more likely to compare themselves with siblings as they develop a sense of self, making them more affected by unequal treatment. Thus, parents need to be fair and explain when differences in discipline are necessary. By adolescence, children better understand nuance and may be less affected by unequal treatment, especially when it reflects individual needs.

In the end, research suggests that what children need most isn’t perfect equality. It’s the confidence that both the warmth and the rules apply to them consistently and without favor.