This is an interesting article I found on Yahoo! News. The study suggests that marriages are 20% less likely to end in divorce if the husband is close to his in-laws. Inversely, marriages where women are close with their in-laws are 20% more likely to end in divorce.
Michigan psychology professor Dr. Terri Orbuch conducted this longitudinal survey study over the past 26 years. She used over 300 couples between the ages of 25 and 37 located in the Detroit-metro area.
Dr. Orbuch believes the difference is that in a close relationship between in-laws and the wife results in increased meddling. As for the men she states, "identity as as a father and a husband is often secondary to their identity as a provider. As a result, they don't take what their in-laws do or say so personally. "
The independent variable is close relationships with in-laws, whereas the dependent value is success of marriage. Cross-lagged surveys described in the book are rare so this is an interesting one to pay attention to. However, it doesn't sound like there are many other studies that are like this so maybe it would be good to take this with a grain of salt. Another reason to be skeptical is that it only surveyed people in Detroit, possibly only reflecting regional beliefs towards marriage.
Cultural changes also change the effects on marriage as what was considered "close" to in-laws may not be considered "close" by today's standards. At any rate, the survey does not say if relationship have gotten closer between husbands and in-laws or have declined which would be a good area of future research.