Monday, August 10, 2015

The decline of marriages in the USA

The United States government likes to keep an eye on its populace, as we all know, and they keep track of numbers, as well as people. Sometimes, they release these numbers, so that we can have a look at them.

The National Center for Health Statistics, which is part of the Centers for Disease Control and Prevention, is an organization that regularly releases its data, particularly those compiled in the National Vital Statistics System. One such dataset that might be of interest is that on Marriages and Divorces.

This dataset has two tables (one for marriages and one for divorces), each provided with a convenient breakdown by state. It covers the years 1990, 1995, and 1999-2011 inclusive; and the data are rates, expressed as "per 1,000 total population residing in area."

If we simply average the data for the whole country, the graph looks like the following. Basically, the divorce rate has remained approximately constant, while the marriage rate has decreased during the current century. The actual number of marriages per year, across the country, decreased from 3.1 million in 1990 to 2.1 million in 2009-2011.

We can now look at whether the marriage trend is consistent across all of the states. As usual, we can use a phylogenetic network as a form of exploratory data analysis, to compare all of the states in a single diagram. I first used the gower similarity to calculate the similarity of the states based on the marriage rates for all of the years. This was followed by a Neighbor-net analysis to display the between-state similarities as a phylogenetic network. So, states that are closely connected in the network are similar to each other based on their marriage rates, and those that are further apart are progressively more different from each other.

The states are neatly arranged in the network in decreasing order of marriage rate from top to bottom-left. I have labeled only the those states with the highest rates.

The result for Nevada surprises no-one who has seen the honeymoon behavior of Americans — the high rate refers to those visitors getting married in Las Vegas, the self-proclaimed "Entertainment Capital of the World". The claim itself may be doubtful (Paris, for example, gets more tourists per year), but the large number of non-residents getting married in Las Vegas is not in doubt. Similarly, Hawaii is a well-known holiday destination for honeymooners, some of whom don't get married until they get there; so this rate does not reflect the behavior of the locals alone.

However, for the other labeled states the rate does seem to reflect the behavior of the residents. It is an interesting mix of states from around the country, although several of the states are from the South, while others have a large Mormon population.

Finally, we can look at whether the decline in marriage rate is repeated across the states. I have plotted the data only for the five states with the highest rates. Note that the vertical axis is on a logarithmic scale.

You will note the steep reduction in the number of people traveling to Nevada to get married, but not so for Hawaii, which has actually increased somewhat. The other states reflect the fact that there has been a general decline in marriage rate throughout the USA since the turn of the century.

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