Be careful with terms like "irresponsible parents" and "family planning". Some people plan to have kids close together in age.
For what it's worth, however, I have this to offer:
* My oldest son (age 5) can read at a second grade level and can do addition.
* My next son (age 4) can read at a first grade level and can also do addition.
* My next son (just turned 3) knows all of his shapes (including the difference between octagons and hexagons), recognize and count all numbers from 1-20, and knows all his letters and their sounds.
* My next son (almost 2) knows all his colors, can count to ten, sing his ABCs, and knows all his animal sounds.
* My last son (7 months old) just figured out sitting up unsupported, and can babble on his level.
Now keep in mind that my little guys also love Pixar movies, playing LEGO games on their Wii, dressing up as superheros, and hiking so they have managed to learn all these things while having a well-rounded and balanced childhood.
The reason these boys know as much as they do and are so well behaved is because my wife and I have taken the time to know their hearts and minds and have invested countless precious hours teaching and training them to be men.
So I want to make sure you understand that just because a family may have children close in age, it is no way has any bearing on their intelligence. My experience has shown me that the complete well-being of children directly correlates to the amount of time and love their parents invest in them, no matter how many siblings they have or far apart their ages may be.
Irresponsible parenting does not mean having many kids close in age. Irresponsible parenting simply means that you aren't willing to offer the time and love your kids need to thrive.