Gifted Issues Discussion homepage Forums General Discussion Question about siblings and giftedness

My 6 year old daughter scored 149 FSIQ on the WISC-IV by a private tester. My 9 year old daughter was tested by the school psychologist (for IEP qualification purposes for dyslexia) on the WISC-IV and scored dead average.

I thought I had read somewhere that siblings (same parents) are usually within a certain range of each other. Just wondering if anybody else has children with such a drastic difference in scores.

That statement about siblings usually being between 10 (or sometimes it's stated as 15) points of one another is one of the silliest things in the giftedness memosphere, IMNSHO, in that it's so often interpreted to mean "so if you have evidence from testing that this is not true of yours something MUST be wrong with the testing". Full siblings share, on average, half their DNA - the same fraction they share with each parent. It may be that siblings tend to have closer environments to one another than they do to their parents, increasing the correlation beteen siblings' IQs, but the way this statement is sometimes made you'd think there was magic attached to being siblings...

What do *you* think in the specific case of your children? Do you see differences in their thinking that seems to support the idea that they have such different IQs? If you don't, it may be that one of the tests was a freak result (or indeed that both were somewhat out, but in opposite directions).

I have fraternal twin boys-- one is deep into PG territory and the other is HG, with much more than a 15-point spread between them. My HG son has dyslexia, which it sounds like your dd does as well. I think the dyslexia depressed his scores, as I think he is closer to his brother in many areas that the WISC reflects. But I do believe their scores pretty accurately affect their abilities-- my dyslexic son is not PG. Their sister, who does not have dyslexia, will probably have a similar IQ to my PG son, but we don't have a reason to test her yet. So, I think the idea that the scores are pretty close to each other might be true-- but probably won't be if a LD is involved.

Does dead average mean her FSIQ is right at 100 or it's up around the higher end of "average" on the bell curve? Were her subtest scores consistently "average" or did they vary wildly? If your dd is struggling with reading issues, and if the subtest scores show variation, it's a good idea to look at them in detail to determine if visual processing or other challenges might have impacted some of the subtest scores.

I am sure there are genetically related siblings who's IQ varies by more than 1 SD from each other - my guesstimate is based on knowing quite a few families who have one child who qualifies for our district's HG magnet program and sibs who don't qualify (IQ is one of the bars that has to be met to get into the program). I've also known a few families where their children were admitted into the program in Kindergarten, the kids didn't perform amazingly so they were retested a few years later and were taken out of the program (by the district) because the new ability testing came in lower than their 5-6 year old testing. That doesn't mean your 6 year old dd's testing isn't accurate - it just means it's *possible* it's inflated. One of my dds had a HG score on the WPSSI at 5 years old but has been tested twice since (WISC-IV and WJ-III Cog) and on both of those tests, at a later age, she is clearly about 20 points below where her early testing put her - which, for her, fits better with what my gut feeling has always been and with how she performs in school.

So.... my suggestion is, pay attention to your gut feeling. For your older dd, take a good look at her subtest scores. Don't focus on whether or not sibs should be close in ability range, just focus on each child, does their individual data make sense, do you need more/better data etc.

Best wishes,

polarbear

My younger son's GAI, obtained when he was 7, was 153. The older one's GAI at age 7 was 106. The older one has dyslexia and I know it (along with some vision and auditory issure) depressed his scores that first time around. Five years later his GAI was 140. I firmly believe that he is at least as intelligent as his younger brother and the dyslexia is messing with the scores.

I take the within 10 points of each other thing to mean that if you're seeing a large difference in IQ (particularly a 2+ standard deviation difference) between siblings that isn't explained by some obvious reason, it might be a good idea to see if the one with the lower IQ has a learning disability and/or sensory processing problem that is interfering with things.

My DS and DD follow the sibling "rule" about IQ but their cognitive abilities are apples and oranges. They are within 10 points of each other's GAI but they are almost mirror images of each other in IQ. DD8 is DYS based on her VCI and DS11is HG in PRI and ADHD/dysgraphic. They each scored exactly the same on the other cognitive ability index, PRI/VCI respectively.

You would think they would be more "alike" based on their numerically similar IQs and genetics but they are almost different species in the way their minds work. Even when the numbers are "close" they can be as different as night and day!

From the Linda Silverman article, What We Have Learned About Gifted Children
30th Anniversary 1979 - 2009

Brothers and sisters are usually within five or ten points in measured ability. Parents' IQ scores are often within 10 points of their children's; even grandparents' IQ scores may be within 10 points of their grandchildren's. We studied 148 sets of siblings and found that over 1/3 were within five points of each other, over 3/5 were within 10 points, and nearly 3/4 were within 13 points. When one child in the family is identified as gifted, the chances are great that all members of the family are gifted.

This still leaves a significant number of siblings (25%) who are more than 13 points apart. I've only had my 2e kid tested so I don't know how the siblings compare.

At first, receiving dd#2's WISC results were a bit of a shock, but now it's starting to make sense and I feel it is representational to what I see in her. Granted she's not doing 7th grade algebra or anything but she seems to pick up math concepts quickly and easily... Her WMI score doesn't surprise me and her VCI is perhaps lower than what I expected. When I told my sister and a close friend the results they weren't the least bit surprised. She probably gets it from dh who is an engineer and his parents were both nuclear engineers.

With older dd who is dyslexic, she has had 2 WISC tests done. The first one done 2 years ago was higher by about 1SD and with significant scatter. The one done this year by the school psych had a lower VRI (40% lower) and PRI but slightly higher PSI and WMI. Her subtest scores ranged from a 15 to a 4 and her FSIQ was right at 100. I feel there is more going on with her than dyslexia, perhaps ADHD, CAPD, etc, I'm really not sure. She has a hard time comprehending basic directions at times. (link to a post I wrote in regards to her testing http://giftedissues.davidsongifted....70/Qualifying_for_an_IEP.html#Post130670)

I do think she is a little more than just average cognitively (but not as high as younger dd). She tends to form friendships with kids in the G&T program, and their parents have told me that I should test her for the G&T program (they don't know about her WISC testing) and that she would do well there.

Yep, dead average 100. Psych wouldn't even look at or consider the scatter. She was really no help. First wisc was administered by a student studying for a bachelors and just noted significant scatter and the rarity of it.


Just curious as to how a score could be inflated. Not offended at all, just wondering as I have little knowledge of how testing works.

That is what I was told about siblings as well. A significantly high percentage lay within 13 points of each other. They did say that the higher the score of one sibling the greater of a chance that number of 13% would be greater as well. But not a huge difference. I was also told, if there is a big gap between the siblings a LD is very likely, which would depress the score.

All this being said I am sure there are kids that are gifted with a sibling that is not. I am just assuming it would happen more when the gifted child is in the MG, HG range. To have a crazy, out there PG child and then one that is of average IQ with NO LD....might not happen too frequently.