Cassar, M. & Treiman, R. (1997). The beinnings of orthographic knowledge: Children’s knowledge of double letters in words. Journal of Educational Psychology, 89, 4, 631-644.

Cassar and Treiman conducted a research study on the knowledge of double letters in English. The study showed students from kindergarten through college. Their interest in the study was to find out how orthographic knowledge plays a role in the letter choices of young children’s created spelling.
Does the knowledge that young children have about the postitions in which double letters occur and about which letters are doubled help in their invented spelling, or is this type of orthographic understanding a late addition to the knowledge base?
The dependent variable was the accuracy when identifying if a word is real or not based on the position of the consonant or letters.
The independent variables were the position of the "doublet" or orthographic convention and the grade level of the student.
The methods for the experiments were based on whether students were shown nonwords with double consonants. Depending on the position of the consonants, the students were asked if the nonword could be an English word, not if it could be a word from another language.
The other experiment studied phonological environment, postiton, and identity of doubled consonants. The students were shown a nonword and given the pronunciation. They were then to choose the best spelling of the word based on what they had heard, not seen.
There are two categories of nonword pairs. The first group had a short vowel and the second had a long vowel. The correct answer was the nonword spelling that correleated to the correct pronunciation.
Results concluded that Kindergarteners were able to identify the correct spelling of the nonwords when the "doublet" was in the correct place. This shows that the students had generalized that words do not begin with "doublets" or contain double consonants that they are not familiar with, for example, kk or bb. As early as first grade, the students were able to identify the correct position of "doublets" in words.
Both sets of subjects conditions tended to choose single consonant spellings more often than the doublets, resulting in more long-vowel words than short-vowel words' spellings being chosen correctly.
This study shows us that with exposure to print, young children are able to generalize correct spellings. Even when students are unable to read independently, they are forming a knowledge of how letters work together and are ordered to form words. Exposing students to print is the key in the early grades in order to develop this understanding from the beginning.