Complex families are a big driver of the increase, said paper co-author Robert A. Pollak, a professor of economics at the Olin Business School of Washington University in St. Louis. Parents' wills may favor biological children over stepchildren, for example, or children from their current marriage over those from a previous one. "Contact seems to make a difference here," he said.
Previous studies have offered other ideas about why a parent chooses to favor one child over another in the will; notably, to give a bigger share to the least well-off child, or as payment for care and attention.
Often it's less about sentiment than practicality. It's not unusual to see imbalances when parents are planning to provide for a special-needs child, for example, or passing on a family business when the kids have varying levels of interest and involvement, said Kevin Meehan, a certified financial planner based in Itasca, Illinois.
Other times, parents want to equalize gifts they have already made, he said — such as paying for one child's education through medical school when another only pursued a bachelor's degree, or funds offered to buy a house, get married or start a business.
Regardless of parents' reasoning, unequal bequests can lead to family and legal woes, said Larry Lehmann, president of the National Association of Estate Planners and Councils. Slighted kids may challenge the will, he said, alleging you had diminished capacity to make that gift, or that the sibling with the bigger share exerted undue influence.
Siblings are at the heart of most will disputes, accounting for 44 percent of cases, according to a 2014 survey by UK law firm Seddons. Claimants cite unequal distributions in 32 percent of cases, and another 46 percent, said it was because "they did not get what they were promised" or "someone took everything that was promised to them."