First of all a big thank you to the 四年生 and 五年生 for their comments on my first draft. They have been vital in my ammendments for my analysis of the role of katakana and its development.
In my first draft I, mistakenly, believed that katakana was essentially an "Occidentally driven form". This was incorrect. It was only post-WWII that katakana was began to be associated with western ideals. For example prior to the 1950's kanji and katakana were used for official documents in a way the reflects the current usage of kanji with hiragana. I believe that this transition or association of katakana to occidental influence post WWII is strongly linked to the cultural impacts that the western world, most importantly the United States, have made on Japan.
As per the original instructions katakana can be categorized into three different categories based on purpose and intention, onomatopoeia, loanword and emphasis. Of the three, perhaps the most obvious examples are onomatopoeia. Numerous example include stomach grumbling
"ルルルルルルル" (Naruto) and laughter
"キシシシ" (One Piece). Though writers have the flexibility to chose either hiragana or katakana to express onomatopoeia, in texts targeting shounen and seinen audience onomatopoeia is predominantly katakana. This raises in interesting question on the role of katakana in developing a Japanese onomatopoeia. After a little research, the use of the Japanese language to mimic symbolic meaning is rather complex. The sound system that is prescribed to create meaning is far more extensive that just onomatopoeia.
For example make unheard sounds that convey meaning such as "キラキラ" (One Piece-when Nami sees money) is not onomaetopoeia but is conveyed with katakana in a similiar fashion to westen concept of onomatopoeia.
Initially I thought that "either the concept of onomatopoeia, or at the Occidental definition of one, did not exist or alternatively the native written mediums were not compatible with present day onomatopoeia" now I see a much more complex interplay that requires much more research that what I have made. Nonetheless it proposes a fascinating topic to explore the development of language in the Occident and the Orient.
Loan words are perhaps the easiest of the three to categorize. They are words taken from other languages that have no Japanese equivalent. For example words such as,
コパーブラウン and
ザ・エルダースクロールズ, スカイリム, have modern origins, consequently they are written in katakana. In a way the loan words are the easiest to explain. But to do so we must return to the origins of katakana once more.
Katakana is an infusion of the Japanese aural language systems, particularity the use of the phonetic mora system, and the characteristic representation of form from the reduction of Chinese origin kanji characters. So to an extent the katakana system itself represents a blend of foreign and local language systems. So it makes sense that these new 'foreign words' to be expressed with the katakana system that is based on the non-native Chinese characters.
Lastly are the words for emphasis . For me the most striking example were
ヒップ & キレイ. Both these words have Japanese equivalents. So why were they written in katakana? In my opinion, the use of katakana for emphasis reveals, more strongly so than the other categories, the impression of katakana by the Japanese paradigm. The use of katakana post-WWII had casual undertones that separated itself from a more traditional Japan. In a way the use of katakana embodies the flavors of modernity, informality and even a degree of cosmopolitanism. The role of katakana in modern Japanese, to a degree, represent the development and globalization of Japan since post-WWII. The use of katakana has a distinct flavor that is distance from the more tradition hiragana and like how Japanese culture has development to fit 21st century lifestyle, the use of katakana is an adaptation of the Japanese language to the rapid process Japan and the rest of the world has made in paradigmatic shifts.
The use of katakana in textbooks have been primarily limited to loanwords. As katakana for emphasis and onomatopoeia are part of the casual portion of the Japanese languagethere are no fixed rules for its structure. Hence such use of katakana would not be included in textbooks as it would imply a structure than simply is non-existent. Another point is that textbook content is much more formal hence casual use of katakana would seem reather out of place.