In fact, generally, students lose the most skills in mathematics. There might be many reasons for this, but largely it may be because there's no mathematical equivalent to a public library for allowing students leisurely interaction with math in the summer. Students lose on average an equivalent of 2.6 months in mathematical skills every summer!
Reading seems to vary based on income levels (which probably could be better called instead “opportunity levels”). Students in middle-income households, on average, actually make slight gains in reading during the summer! (Again: think public libraries and lots of free time for your child to read what he/she enjoys reading) Low-income students, however, lose close to the same amount in reading as in mathematics with again more the than two months lost. These students' parents may have less time to take them to the library, less education and comfort reading to their children, less disposable income to spend on books, and the least knowledge about why it's important to encourage reading them during the summer. Every summer the gap between the reading abilities of low-income and high-income students increases.
<Clearly, if your child is already behind or has struggled so hard to catch up with his/her peers, you can't afford to simply passively allow the usual summertime regression. You all worked too hard to let it just go by the wayside. Your child's self-esteem deserves better. Your child's academic career requires better.