It takes extreme motivation to cross Mexico in the hands of human smugglers, the dangerous journey taken by most of the Central American families and children who have arrived in huge numbers at the Texas border.

Most of the migrants cite what are often called “push” factors as the reason for leaving their home countries. Because of gang-related violence and dire poverty, they say, they were desperate to leave and came to the U.S. essentially as refugees.

While every family has their own motivations for overturning their lives and trekking across the Mexican desert, economists say the current wave of illegal immigration from Central America was predictable, to an extent, and for different reasons than the migrants might identify.

It is true that Honduras, among the other "Northern Triangle countries" of Guatemala and El Salvador, is suffering high levels of crime related to drug trafficking and gangs. San Pedro Sulas, the second largest city in Honduras, holds the distinction of “murder capital of the world.”

In 2012, there were 90 murders per 100,000 people in Honduras, the highest homicide rate in the world, according to the United Nations Office on Drugs and Crime, compared with only five in the U.S. In some developed nations, such as Germany, the rate is near or below one.

But crime is only part of the story. Early indications are that crime may have fallen in 2013, when Honduras-U.S. migration began accelerating. And immigration from Honduras is picking up as Mexican immigration has declined -- even though Mexico itself has seen an outburst of drug- and gang-related violence and has a murder rate four times that of the U.S.

The difference might have to do with changes in income, which economists think affects immigration decisions in two ways — one that discourages migration and another that boosts it. First, income growth in the home country can slow emigration by narrowing the gap with the destination country, making the move seem less attractive. Or, growing income can give some people the resources they need to finance a trip.

Migration often is too expensive for the poorest of the poor. One Honduran woman who entered the U.S. in June told the Washington Examiner that she paid a coyote, as human smugglers are called, $3,500 a head for her and her toddler daughter to take them across Mexico. That sum, $7,000, is almost three times the yearly per-capita income in Honduras.

Despite the gang-related violence, both Mexico and Honduras have seen significant progress in recent years. The countries have averaged roughly 3.5 percent annual growth in their gross domestic product since the recovery from the global financial crisis began in 2010, well above the United States' 2.3-percent mark, according to the World Bank. But Mexico is at a much further stage of development, with an output per capita roughly four times that of Honduras. Similarly, fertility, another determinant of emigration, is falling fast in both countries. Mexico is near two children per woman, the replacement rate required to prevent the population from shrinking, and Honduras is slightly higher at roughly three children per woman.

In other words, faster development south of the U.S. may have decreased Mexican immigration by narrowing the income gap, while it may have increased Honduran immigration by reducing poverty and making travel affordable for families who previously lacked the means to come to the U.S. but still would benefit from the greater opportunities and higher income potential.

Similar trends influenced Irish and Italian mass migration patterns in the late 19th century. Following the Potato Famine exodus, Irish immigration slowed in the late 1800s as “Irish wages rose strongly compared with those in Britain and the New World,” wrote economists Timothy Hatton and Jeffrey Williamson in a study of immigration decisions. “In Italy, by contrast, gradually rising real wages and the cumulating emigrant stock abroad both served to progressively release the poverty constraint and emigration rose strongly as a result,” and the mass Italy-U.S. migration resulted.