Seven is a magical number for humans. Our skin cells regenerate every seven days; our skeletal cells replace themselves every seven years; and when humans divorce, chances are good that they’ll do it after — you guessed it — seven years of marriage.

According to 2011 U.S. Census data, the median length of first marriages that end in divorce is seven years. Most people call it “the seven-year itch,” after the 1955 Marilyn Monroe movie of the same name, which popularized this theory that the passion fades before that eighth anniversary rolls around.

But why seven years and not six or eight years? Marriage experts often blame accumulated stress from struggling through the lean first years of marriage and raising small children — after seven years, they say, the kids are old enough to be in school and the parents find that they long for something different, something that doesn’t remind them of the turmoa

Still, that doesn’t quite explain the marriages that don’t include children or the marriages that dissolve right after the baby is born, like Angela Zook’s marriage.

Zook, 38, had been with her husband for seven years when the couple decided to call it quits, but says it had nothing to do with their child’s age.

“Our daughter was only 1 year old when my husband left me,” Zook says. “I was blindsided.”

And then there are famous, childless and wealthy couples like Brad Pitt and Jennifer Aniston, who divorced after seven years. You can’t blame that break-up on financial or parenting stress, but that magical number seven still played a role.

So what gives? Following are some of the most interesting pieces of scientific research behind the “seven-year itch” and professional recommendations for helping your marriage survive to year eight and beyond.

Research shows two dips in early marriage satisfaction — In 1999, Dr. Lawrence A. Kurdek, published the results of his marriage research in the journal Developmental Psychology, and showed that there are really two dips in marriage satisfaction within the first 10 years. The first dip comes after four years and the second after seven years. Perhaps the fact that couples are experiencing their second dip in passion and satisfaction after seven years makes them question the stability of their relationship? Kurdek’s research also showed that couples who had children also reported higher levels of unhappiness in the marriage after four and seven years.

Boredom also plays a role in long-term marital happiness — Marriage researchers from the University of Michigan and Stony Brook University found that couples who reported feelings of boredom in their marriage during year seven, also reported less happiness and marital satisfaction nine years later, during year 16 of their marriage. Researchers concluded that feeling boredom or “stuck in a rut” during year seven of marriage “undermines closeness, which in turn reduces satisfaction.”

The chemistry behind lasting love — New research into the chemicals that are produced when we fall in love, get married and raise children with our partners shows that humans typically go through three main chemical phases in the course of a long-term romantic relationship. The first phase produces lustful hormones like testosterone when you first meet someone you are sexually attracted to. Later in the relationship, you feel those almost obsessive “crazy in love” feelings when your brain floods your body with dopamine, also known as the reward chemical. Many couples fall “out of love” once the dopamine rushes subside — usually after two years, but sometimes after as many as four years — so researchers looked at chemicals that were present in lasting romantic relationships. In the third stage, our bodies produce bonding or attachment chemicals that help keep us feeling romance for our partners even after the lust is a distant memory. After sex, our brains are filled with the attachment hormone oxytocin, which helps bond partners for the long-term. This could be a reason why couples who report more frequent sexual encounters with one another tend to stick together longer and claim higher levels of marital satisfaction.

Recommendations from marriage counselors on weathering the seven-year itch:

  • Make weekly dates with your partner and dress up for him or her. Feeling more attractive will increase your confidence and help your partner remember those first sparks of desire.
  • Talk to each other at least twice a day about something other than the kids, your house, your bills, and other routine-life things. Ask your partner to tell you a story from their childhood that you haven’t heard before. Ask their opinion about a new movie or album. Keep getting to know each other so you can find something new to love.
  • Get your own life. Couples who have interests of their own tend to report stronger marriages and higher levels of romantic satisfaction. Find at least three things that you truly love to do and then do them on a regular basis, regardless of your partner’s interest in those activities.
  • Give your partner a sincere compliment at least once a day, even if it’s something as small as, “You look really great today,” or “You always know what to say to cheer me up.” Let them know that they are appreciated.
  • Don’t let the small stuff in your relationship fester. If you have a problem, don’t keep it inside. Talk about it with your partner and then let it go. Resist the urge to bring it up again once you’ve come to an agreement or a solution. Getting rid of the everyday stresses in a marriage can help prevent big blow-ups that lead to hurt feelings and make the idea of single life seem more appealing.
  • Have more positive sex. Yes, life is hectic and sometimes you just don’t feel like it, but the hormones that you release after orgasm help bond you to your partner and make a relationship stronger. One interesting study from the University of Toronto found that people who reported that they had a “positive partner-focused approach” to sex — that is, they were having sex because they wanted to bond with their partner or make their partner feel good — also reported greater levels of sexual and marital satisfaction. Study participants who reported having sex for negative reasons, perhaps to avoid conflict with their partner, reported less satisfaction and researchers believe these negative-influenced sexual acts could actually be more detrimental than no sex at all. So, have sex, but do it because it feels good for you and because it will feel good for your partner — not because you want to avoid a fight!