Americans are contributing twice as much to their adult children as they are to retirement, according to a new study.

U.S. parents spend $500 billion annually supporting and giving to their adult children aged 18 to 34, double the amount they contribute each year to their retirement accounts, according to a new study conducted by Merrill Lynch in partnership with Age Wave.

While  79 percent of more than 2,500 American parents surveyed said that they give at least some financial support to their children, two-thirds of American parents report having sacrificed their own financial security for their children.

Most parents of children 18 to 34 are still supporting day-to-day expenses. The respondents said that these expenses include food and groceries, funded by 60 percent of the parents in the survey; cell phones, funded by 54 percent; car expenses, funded by 47 percent; school, 44 percent; vacations, 44 percent; and rent, 36 percent.

Parents are also helping with big-ticket items: 59 percent said they contributed or expect to contribute to their children’s weddings, and 26 percent to the purchase of a first home.

Most parents, 90 percent, said they were surprised at the cost of parenting, yet 93 percent of the respondents said being a parent is the most rewarding aspect of their lives, and 94 percent said that parenting has been worth “every penny” it has cost them.

In her keynote speech at Financial Advisor's 2018 Inside Retirement conference in Las Vegas, Age Wave co-founder Maddy Dychtwald said that changing gender dynamics, combined with longer lifespans and delayed life transitions among younger people, have made a "sandwich generation" out of today's retirees and pre-retirees, where they often have to care for adult children and their own parents for longer.

Later, at another discussion about preventing failure of retirement plans, a panel of advisors identified dependent, entitled and financially illiterate adult children as a primary threat to their parents' retirement.

The Merrill Lynch/Age Wave survey was conducted in June.