It's long been believed that older fathers are more likely to have children with schizophrenia or autism, and new research finally reveals why. The study, published Wednesday in the journal Nature, finds that a man's DNA continuously mutates with age, and the mutations, when passed down to his offspring, can have highly negative effects. The working theory is that the more mutations in a man's genetic code, the more likely it is that his child will develop abnormalities. Here, a guide to the research, and what it means for prospective fathers:

First things first: How was the study conducted?
The findings were based on the genetic analysis of 78 Icelandic families, all of which had offspring with a diagnosis of autism or schizophrenia. 

What happens to men's genes as they age?
As men age, a variety of environmental factors, such as radiation, cause genetic mutations, and sometimes the mutations are even caused by "mistakes that occur in cell division," says Elizabeth Lopatto at Bloomberg Businessweek. Researchers found that each additional year of life results in an average of two new mutations. For example: "A 20-year-old father transmits, on average, 25 new mutations to his child while a 40-year-old transmits 65."   

Why do mutations lead to an increased risk of brain disorders?
Genes provide the "blueprints" for developing all the proteins needed by the human body to grow and work properly, says Lopatto. While most of the mutations passed along are harmless, a child's mental process is more likely to be affected "because more genes express themselves in the human brain than elsewhere," leading to illnesses like schizophrenia and autism. 

Does this explain why autism is on the rise?
The prevalence of older fathers could definitely be to blame. In developed countries, more and more men are fathering children at older ages, study author Dr. Kari Stefansson tells the Los Angeles Times. "It's very likely to have made meaningful contributions to increased diagnoses of autism in our society." According to the U.S. Centers for Disease Control and Prevention, one in 88 children in the U.S. had autism or a related disorder in 2008, the latest period such data was available. That's a 23 percent increase from the previous period in 2006. 

What about genetic changes in women?
The average number of mutations coming from the mother's side was 15, regardless of age. That's because egg cells are relatively stable, says Benedict Carey at The New York Times. Sperm cells, on the other hand, divide roughly every two weeks, and continual copying "inevitably leads to errors." That the father's age accounted for all this added risk "is absolutely stunning" given the "diversity of the population," Dr. Kari Stefansson, the study's senior author, tells The New York Times. "And it's stunning that so little is contributed by the age of the mother."

So what should potential fathers do?
Men looking to put fatherhood on hold should consider collecting their sperm at a younger age and storing it for use later on, says Alexey Kondrashov, a professor of evolutionary biology at the University of Michigan in Ann Arbor who wrote the study's accompanying editorial. Cold-storing sperm for later use "could be a wise individual decision."

Sources: Bloomberg Businessweek, Los Angeles Times, The New York Times, The Wall Street Journal