What separates these two groups of hitters, besides their stats?

These groups cover all non-pitchers with 500+ plate appearances over the last five years. Each group has over 140 players, totaling more than 200,000 PAs. Group OPS+ is weighted by playing time. The WAR figures are rates per 650 PAs; the ratio between the groups would be about the same if expressed per 162 games.

Assignment to Group A or Group B was not based on any baseball statistic. What was it based on?

Here are their counting stats, per 650 PAs:

To round out the picture, here’s a quick look at the pitchers, divided on the same basis:

This is all pitchers with 100+ innings over the last five years. Each group has over 140 pitchers, totaling more than 40,000 innings. Group ERA+ and OPS+ are weighted by playing time. WAR is the rate per 200 innings.

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Congratulations to Joseph, who determined that Group A is players born in the United States,* and Group B is those born elsewhere.

This quiz grew from wanting to check the old chestnut that “you don’t walk off the island.” Of course, not all foreign-born players hail from the Caribbean area. But more than half of the foreign-born hitters in this study came from Caribbean Islands, mainly the Dominican Republic (46), Cuba (13) and Puerto Rico (13).* And most of the rest came from countries on the Caribbean — Venezuela (48), Colombia (3), Panama (3), Mexico and Nicaragua (1 each).

Anyway … The recent data do show a slightly lower walk rate among foreign-born hitters. But there are even bigger differences:

• Strikeout rate is much lower among the foreign-born — 16.4% vs. 19.4%, a margin of almost 20 Ks per 650 PAs — leading to a 9-point edge in BA despite the same BAbip. Their gains via contact more than offset the lower walk rate, and came with very little loss in the power departments: U.S.-born players averaged one extra home run per 650 PAs, while doubles and triples were even.
  Overall, the foreign-born hitters had a slight edge in OBP and slugging, driven by BA. The rest of the season rates are extremely close.
• Foreign-born players had more defensive value, with an edge of about 0.3 dWAR per 650 PAs, accounting for almost all of their overall edge in WAR. This could be from actual skill, or from a higher concentration at the more valuable defensive positions. It doesn’t seem to be age-related, as the unweighted average of both groups is just under 29.

Foreign-born pitchers had a higher strikeout rate, although by a smaller margin (20.3% vs. 19.3%). This might grow out of their developing years: Since a certain degree of strikeout skill is needed for success, foreign-born pitchers might have had to hone this skill more sharply because they faced hitters who make more contact.

Finally, here’s a longer view of the hitters, covering 1995-2014 and all those with 300+ PAs:

WAR figures above are per 650 PAs, as are the following:

A similar picture on strikeouts and value, but milder.

And now we throw the floor open to comments.

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* Puerto Rico is a U.S. commonwealth whose people are U.S. citizens. But it’s treated as non-U.S. by my data source, Baseball-Reference.com, and there is some logic behind that. No slight is intended.