I disagree with this statement. Intelligence is mostly controlled by your genes, it isn't even disputed in the science community. Has nothing to do with how you were raised. There's a bunch of studies on adopted kids and twins that were raised apart. No one ever likes to talk it about because for certain reasons i won't bring up because this type science has been used to commit some of worst crimes against humanity in modern history. Also IQ doesn't mean successful, being successful has to do with your upbringing and education. Smartest guy in the US is a bouncer after dropping out of college. He grew up in poverty and a broken home.
The 2006 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings.