1. The mental addition task Children will be presented with the 81 possible additions with operands from 1 to 9, with 6 presentations of each operation resulting in 486 trials. Additions will be classified as small when they will involve operands from 1 to 4, as medium with operands between 5 and 9 and a sum up to 10, and as large with operands between 5 and 9 and a sum greater than 10. For each trial, a ready signal (an asterisk) centered on screen will be presented for 250 ms followed by the simultaneous presentation of the two operands separated by the sign +. The operation will be displayed horizontally and centered on screen. The onset of the oral response will stop the timer of the computer using a voice key. The 486 trials with be presented in 6 blocks of 81 trials, with each operation presented once in each block in random order. The experimenter will key the response given by the child, triggering the next trial. In a pre-test, we have already determined that it takes about 1 hour for children to solve the whole set of operations. 2. Working memory assessment In order to obtain a reliable measure of children’s working memory capacities, we will assess children’s memory span using two different tests: - In the classic counting span task (Case, Kurland, & Goldberg, 1982), cards with varying numbers of red targets dots and green dots are presented to children. The cards are presented one by one in series of ascending length, from 2 to 6. After each dotted card, a card with a letter to be remembered is inserted and presented to children during approximately 1 second. Children are instructed to point and count the red dots out loud and to read and remember the letters. When a card with the word “Recall” is presented to child, he or she has to recall the letters in the correct order. When a participants fails to recall the letters on each set of given lengths series correctly, the series of higher length is not presented. The maximum number of letters that a child can recall represents his or her memory span on this task. - In an adaptation of the reading letter span task (Lépine, Barrouillet, & Camos, 2005), children will be asked to name pictures out loud while maintaining letters inserted between two pictures. Series of 2 to 6 letters will be presented and, as for the counting span task, the test will be interrupted when the child fails to recall the letters on each set of a given length. The original version of the test was constructed with letters and numbers but it is important for us to use at least one task without any numerical component. Indeed, children who experience difficulties in arithmetic could be poorly efficient in a working memory task only because numbers provoke anxiety and not because they really have poor memory capacity. 3. Arithmetic skill assessment The TTR (Tempo Test Rekenen; De Vos, 1992) will be used to test children’s arithmetic skills. The TTR is a test consisting of 200 arithmetic number fact problems presented in five rows (one row with addition, one row with subtraction, one row with division, one row with multiplication, and one mixed problem row). Within each row, the problems increase in difficulty. Children are instructed to solve as many items as possible within 1 min per row. The TTR is a standardized test that is frequently used in the Flemish education system and we will have to translate the instructions in French. It is suitable from children from Grade 1 to Grade 6 and only takes 5 minutes. Note that this is not a problem for us that the test is not standardized for Swiss children because our goal is to compare children’s arithmetic skills within our sample and not compared to the general population. These tasks will be performed in three different sessions with two blocks of the 81 additions presented in each session. The TTR will be performed in the first session after the addition task. The counting span task will be performed during the second session and our adaptation of the reading letter span task will be administered during the third session. Each session should take about 30 minutes.