Approximately half of all colleges lure incoming freshmen with appealing ­financial-aid packages, only to slash those aid packages in future years, often by several thousand dollars or more.

Could reputable colleges really be doing a classic “bait and switch”? Yes, says college financial-aid expert Mark Kantrowitz. Research has found that “front loading” raises the probability of matriculation by 48%.

Examples: Freshmen at Touro College in New York receive an average of $10,227 in grants and scholarships, but the average for all undergrads is just $5,521. Freshmen at the prestigious University of Pennsylvania receive an average of $35,539 in aid, but undergrads overall average just $30,217.

Some colleges do warn freshmen that aid may drop, but usually it is a generic statement that aid may change.

Self-defense: When considering any aid package, ask a financial-aid employee, “Will I get the same grants as a sophomore, junior and senior as I do as a freshman, assuming that all else remains unchanged?” If you get a yes, ask for a commitment in writing. If you don’t get a straight answer, this employee probably is being intentionally evasive to avoid admitting that your aid likely will decrease.

In that case, you can visit the Depart­ment of Education’s College Navigator Web site (­CollegeNavigator). Enter the name of a college, then click on its “Financial Aid” tab. This will produce a pair of tables, one labeled “Full-time Beginning Undergraduate Students,” meaning freshmen…the other “All Undergraduate Students.” Look for the lines labeled “Grant or Scholarship Aid,” then compare the average amount for freshmen to the average amount for all undergraduates.

If the gap is greater than $500 to $1,000, there’s a good chance that your aid could decrease after freshman year.