Here is some free, evidence-based parenting advice: Try at all costs not to discipline or otherwise treat your children differently, lest they grow up to resent it and each other.

That is the main finding from a new study by two scholars in the University of Kansas Department of Communication Studies.

Using the retrospective reports of 325 college-age adults, the study found that parental differential treatment toward offspring and family cohesion work together to affect siblings' relationship quality. Specifically, the overall family climate of emotional connectedness helps promote better sibling relationships. The paper is published in the journal Family Relations.

With extreme levels of parental differential treatment, or PDT, however, some positive implications of family cohesion no longer exist for sibling relationship quality.

The authors of the study urged fathers, who often play a disciplinarian role, to be aware of this potential and to avoid it by treating siblings equally whenever possible. If circumstances make that impossible, they wrote, parents should explain why they are treating siblings differently.

"Young adults' retrospective reports of family cohesion, parental differential treatment, and sibling relationships" was published online in Family Relations. Its authors are Weimiao Zhou and Alesia Woszidlo. Zhou recently graduated with her doctorate from KU's communication studies department, where Woszidlo is an associate professor.

Informed by family-systems theory, the authors examined how parental differential treatment affected the relationship between family cohesion and sibling relationship outcomes.

PDT can consist of "differential affection (e.g., parents showing different amounts of love, warmth, and care to their children) and differential control (e.g., parents showing different amounts of controlling behavior such as disciplining, punishment, and blame to their children," they write.

In the case of both mothers and fathers, PDT was found to negatively affect family cohesion and sibling relationship quality, the authors wrote. This is consistent with past research as well.

"But in this particular study, fathers' differential treatment emerged as a more robust moderator, in comparison to mothers, with regard to the strength (of the relationship) between cohesion and sibling relationship quality," Zhou said.

For instance, the authors write, "The present study suggests that fathers who display different amounts of control (e.g., showing different amounts of strictness, blame, discipline, and punishment) toward two offspring is likely to weaken the positive effects of family cohesion on sibling affection."

Family cohesion, Zhou said, "means that families have lots of routines that promote togetherness, as well as parents who try to treat children as equal. Those two factors work together to promote the siblings' relationships."

Treating siblings equally does not always mean treating them exactly the same, according to the authors. Sometimes differential treatment is warranted and needed, they wrote, especially when siblings differ in age or have varying developmental needs.

"It's OK to show differentiation," Zhou said. "It's just maybe that fathers need to provide more information about why they engage in differential treatment to help their children process the reasoning part of PDT, so as to reduce that kind of negative effects … to help the siblings to have pro-social behaviors and more positive interactions."