I think, it depends on individual character and not on gender. My girl is very relationship-oriented, she wants someone talking to her
all the time. My son is more object-oriented, and he likes very creative but solitary play. As a result, my girl is much more advanced in language and related development than my boy. My boy reads earlier than my girl, and he is also ahead in math - but it is because I worked with him much more. Why I did it? Because I was worried about his language delay.

I think, we can generalize this to boys and girls. Often, boys are slower not because they are boys but because they prefer more solitary play, and are less interested in talking. With less talking, also the development of logical thinking can be slower. But it is not bad; it is outweighed by the development of creativity

I open Word, make font size 90, and write funny sentences like "cow is climbing tree, horse is flying in the sky, [child's name] is stronger than Dad" and so on. But I do it for fun, and I never use any pressure, rewards etc. I think that if the toddler can read than no further pressure is necessary. She is already so far ahead, and we can afford the luxury of unschooling mode.

Just now, I googled
"Nurture Assumption" pdf
and I managed to get a free copy. The second link. I think, you should hurry

Do you have some threads/links/blogs about this topic?

Well, I have. http://forum.brillkids.com/coffee-corner/about-the-happiness/
Now I find it hard to read.
There are many similar threads.

Few years ago I thought that parenting is like cookbook. You do this and that, and you get that
But our children belong to themselves. They will find their own way. Your parents raised you in their own way and environment - but you found another preferences anyway.   And, probably, they ensured you some education. Would you be happier if you would have not read all these books in your childhood?

Once I read a good book about underachievement. Now I tried to find it and found this

Underachieving Gifted Child: Recognizing, Understanding, and Reversing Underachievement
http://www.amazon.com/Underachieving-Gifted-Child-Understanding-Underachievement/dp/1593639562 The

There are many other books if you search amazon for keywords "underachievement child".

I was surprised to browse following book tittles

What to Do When Good Enough Isn't Good Enough: The Real Deal on Perfectionism: A Guide for Kids

The Gifted Kids' Survival Guide: For Ages 10 & Under

Bright Minds, Poor Grades: Understanding and Motivating your Underachieving Child

Why Bright Kids Get Poor Grades and What You Can Do about It: A Six-Step Program for Parents and Teachers, 3rd Edition
(this is what I read few years ago; it is about various aspects, mainly in family, that can cause underachievement.)

Social/Emotional Issues, Underachievement, and Counseling of Gifted and Talented Students (Essential Readings in Gifted Education Series)

When Gifted Kids Don't Have All the Answers: How to Meet Their Social and Emotional Needs

Keys to Parenting the Gifted Child

Giftedness, Conflict, and Underachievement

and more.

I wish you a very good luck!

I was satisfied with this answer

http://blogs.scientificamerican.com/guest-blog/2011/03/07/you-can-increase-your-intelligence-5-ways-to-maximize-your-cognitive-potential/

Summary.

1. Fluid intelligence is trainable.
2. The training and subsequent gains are dose-dependent—meaning, the more you train, the more you gain.
3. Anyone can increase their cognitive ability, no matter what your starting point is.
4. The effect can be gained by training on tasks that don’t resemble the test questions.

.... These five primary principles are:

1. Seek Novelty
2. Challenge Yourself
3. Think Creatively
4. Do Things The Hard Way
5. Network

===

I think, typical mind training games do not offer these 5 principles. The result depend on inner and outer challenges and individual approach might work better than mass approach

and I like to read this type of articles
http://www.brainy-child.com/article/thinking-skills-for-children.shtml 
http://www.brainy-child.com/article/critical-thinking-skills.shtml

As non-English parent I can share my material
http://library.brillkids.com/download.php?cid=6&tid=&lid=104&fid=9287
which is based on Reading Bear.

I think, most of the parents use several approaches at the same time, and search for the preferences of their children.

according to video, 12 is not the number of participants; 12 is the number of non-structured learners. Author says that these were harder to find. So I guess that structured learners were much more.

Korrale4kq,
youtube search: Are home-schooled children smarter? Concordia University
the first video

"Parent training video for using dialogic reading techniques with videos."

It is similar to dialogic reading but the object is video instead of book.
Well, I had already revealed it by myself, but this video is exellent.

<a href="http://www.youtube.com/v/b6ZJRFwdF3c&rel=1" target="_blank">http://www.youtube.com/v/b6ZJRFwdF3c&rel=1</a>

TheyCan ,

when my son was two years old, I created powerpoints with family pictures. I do not know if he got a benefit but he liked them
For example: picture with A and tractor. What is it? It is a tractor. What A is doing? He is sitting in tractor. Which color is tractor? It is read. One word per slide, and picture after each sentence. I also prepared tons of other small ppt files on simple subjects.

Great success was this file, with animated clipart. I started with the easiest words and added few words after every few days.
http://library.brillkids.com/download.php?cid=6&tid=&lid=104&fid=9287

I allowed him to operate powerpoint; this definitely kept his interest. He opened and modified files, and changed settings. He had his own profile, all files were saved as "read only", and I often updated the content of his folder. So I kept control on files and his learning. I kept his ppt files very short because usually he watched just the first few words. He choose the files, as he could read their names.

----

A bit of general

There is a quote

"Parents often do not become believers in temperament until after the birth of their second child. Before this time, their child's behavior may be seen as a simple and direct outcome of their upbringing, "a tribute to" or "the fault of" the parents. With the second child, management strategies that worked well with the first child may not be effective. Problems experienced with the first child (in feeding, sleeping, coping with strangers) may not exist with the second, but new problems may arise. Such experiences suggest strongly that "nature" as well as "nurture" influences child development, that children differ from each other from very early in life, and that these differences have important implications for parent-child interaction. A number of these individual differences fall under the rubric of child temperament, the subject of this chapter. Here, we define temperament as individual differences in reactivity to internal and external stimulation, and in patterns of motor and attentional self-regulation."

http://www.bowdoin.edu/~sputnam/rothbart-temperament-questionnaires/cv/publications/pdf/1995_Child%20Temp%20and%20parenting_Sanson-Rothbart.pdf

Well, I think often about how different my children are. They have nothing in common except their parents

My first kid is "early left-brained" - two years ahead her peers intellectually. she likes, she prefers to follow rules and instructions. she likes to behave well. she likes to watch and listen, and follow. She likes all kinds of EL.

My second one is not interested in rules and instructions. His main interest is imaginative play. At every free moment he takes toys (or non-toy things) and plays with these objects. His toys drive, fly, meet aliens, eat pizza, discuss daily events, relationship and other issues. They express their emotions loudly. He had discipline issues in childcare. He likes to play alone. He is very persistent and strong-willed. He is often singing, and it seems (to me ) that he is composing his own songs.

I do not think that one of them is more advanced. My first kid is very "handy" or "comfortable" to all kinds of educators. In her further life, she might become an expert but probably she will avoid leadership because she does not like to take risks, to resist and to look for her own way.

My second one seems to be similar to his grandfather who is very active, laborious, and he also always was leader and wealthy.

When I watch my son playing I sometimes remember texts like these
http://www.psychologytoday.com/files/attachments/1035/imaginary-worldplay-indicator-creative-giftedness.pdf

As he likes to play alone, he is at risk of late talking and little knowledge. I keep his knowledge and other skills in a sufficient level.

Well, I wanted to say that children are very different, and EL is not the only way to happiness

Currently I am reading newsletters of TestingMom, and she says:
"Constantly feeling guilty and inadequate is part of being a parent. If you aren’t worried that you aren’t doing enough for your child, you’re probably not paying close enough attention! "    (It follows that you should feel guilty also if you are not feeling guilty because it means that you are guilty.)

I started EL when my first kid was 1.5 years old, and she already was very advanced. With my second kid, I started math at 3 months and other flashcards at 7 months. When he was nearly 1 year and 10 months I found that something goes wrong; he was not advanced at all.

Maybe you remember my posts half year ago here. My son was not delayed; he was just average. Now he is advanced, I think.

Now he is 3 years and 8 months and I can say with responsibility: yes, EL did not work until his 1.10 years. EL without interaction and without a lot of talking does not work. It is waste of everything. Our EL started to work when I discontinued the use any ready-to-use materials and started to observe what he is interested, and what works and what does not work, and started to prepare original materials for him.  A lot of thinking about his personality and interests.

For example, math. With my first kid, I started simple quantity recognition when she was 1.5. Later I did also Doman math and LM but she was not much interested. Although she was able to count to 12 in two languages until she was 2 years old, she understood just "3" since she was 2.5 and "4" since 3.5. At 4.5, her math ability exploded and now she is rather advanced.
With my second one, I did physical math flashcards since 3 months, LM since 7 months, he watched both LM courses. Since his 2 years I show him math every day; mach more as I did it with my first kid. And he is exactly at the same level as my first kid was when she was 3.8. He follows the same steps of development as my first kid did. So I think that at least his 1.5 years of EL were just waste of time.

I did also a lot of art and EK flashcards. I do not see of feel any results. I see results from my other, later activities - dialogic reading and daily conversations.

Best wishes

Thank you! Very useful links.

(I can read Russian too )

upd.
we are going to try all of these  

Turn Salt Water into Drinking Water
http://www.education.com/activity/article/Take_salt_out_of_salt_water/

Make a Butterfly Habitat
http://www.education.com/activity/article/butterfly_habitat_first/

Natural Easter Egg Dyes
http://www.education.com/activity/article/easter-eggs-natural-dyes/

How Old is That Tree?
http://www.education.com/activity/article/How_Old_Are_They/
also: measure height of tree

Pressed Flower Cards
http://www.education.com/activity/article/mothers-day-pressed-flower-card/

Kokedama
http://www.education.com/activity/article/kokedama/

Helicopter Seeds
http://www.education.com/activity/article/helicopter-seeds/

http://www.education.com/slideshow/food-science-fourth/electric-spoon/

Homemade Barometer
http://www.education.com/activity/article/pop-bottle-barometer/

Make Spider Web Art!
http://www.education.com/activity/article/Collect_Spider_Webs/

Design a Great Glass Xylophone!
http://www.education.com/activity/article/musical-great-glass-xylophone/

Make a Water Magnifier!
http://www.education.com/activity/article/make_a_water_magnifier/

Levitating Ice Cubes
http://www.jumpstart.com/common/levitating-ice-cubes

Invisible Ink
http://www.jumpstart.com/common/invisible-ink

I am participating in a project where my task will be to fill one page per month with something interesting kids (age 6+) can do in nature or with nature materials

Project will start in January.
As we do sprouting of this style <a href="http://www.youtube.com/v/OSt0DmAPDm4&rel=1" target="_blank">http://www.youtube.com/v/OSt0DmAPDm4&rel=1</a> , this will be our material for January.
We also do sunflower sprouting, with another method.
We have snowy winters here (N Europe), so part of activities should be indoors.

In spring, I would like to build bird observation shelter.
In summer, we will grow plants and observe wildlife.
I am already collecting leaves for something like this http://klab.lv/users/kihelkonna/384796.html#cutid1

Can you, please, recommend me ideas, experiments, links? I know that many of you have astonishing experience

TheyCan,
your learning routine seems to be very screen-based. I would recommend to focus on dialogic reading (not just reading aloud; search google and youtube) and all kinds of conversation.
I recommend also this article
http://pediatrics.aappublications.org/content/124/1/342.full.pdf+html
good luck