## Introduction

Experiment | N (counting direction) | Stimuli | Lateralized encoding | Lateralized response | Number concepts |
---|---|---|---|---|---|

1. Naming numerosity | 21 (left–right) 13 (right-left) | Visual dots | Yes | Yes | Yes |

2. Length production | 16 (left–right) 13 (right-left) | Digits | No | No | Yes |

(a) | |||||

(b) | Yes | Yes | |||

(c) | No | Yes | |||

(d) | Yes | No | |||

3. Weight reproduction | 60 (left–right) 44 (right-left) | Weights | No | Yes | No |

## General method

## Experiment 1: numerosity naming

### Participants

### Stimuli and apparatus

^{1}Resulting dot clouds were shown in black on white background on a 19-inch display with 1280 × 1024 pixel resolution (landscape orientation), located on either the participant’s left side or right side, while the keyboard was always straight ahead. Presentation of stimuli was software-controlled and responses were made verbally; the experimenter noted these estimates after each trial.

### Design and procedure

### Results

^{2}= 0.162. Figure 1 shows numerosity estimates on left and right screens separately for the two groups which differed in the direction of their counting preferences. The 21 left-to-right counters gave marginally smaller estimates when dot clouds were presented on the left (M = 26.036 dots, SE = 1.170) compared to the right screen (M = 28.619 dots, SE = 1.103), t(20) = − 1.85, p = 0.079, Cohen’s d = 0.44. The 13 right-to-left counters gave instead larger estimates when dot clouds were presented on the left (M = 28.596 dots, SE = 1.487) compared to the right screen (M = 26.452 dots, SE = 1.402), t(12) = 2.59, p = 0.024, Cohen’s d = 0.53.

## Experiment 2: length production

### Participants

### Stimuli and apparatus

### Design and procedure

### Results

^{2}= 0.185 (see Fig. 4). Specifically, left-to-right counters produced significantly shorter lines on their left compared to their right side (M = 522.731 vs. 539.617 pixels, respectively, with SE = 14.609 and 15.803), t(15) = 2.57, p = 0.021, Cohen’s d = 0.35. Instead, right-to-left counters produced only numerically longer lines on their left compared to their right side (M = 513.523 vs. 502.505 pixels, respectively, with SE= 16.207 and 17.532), t(12) = 1.15, p = 0.271.

^{2}= 0.062. Descriptively, left-to-right counters reproduced shorter lines after seeing numbers on their left side compared to their right side (M =524.561 vs. 539.144 pixels, respectively, with SE = 18.218 and 16.633), t(15) = 1.83, p = 0.088, while right-to-left counters produced similar line lengths in the left and right conditions (M =497.335 and 498.581 pixels, respectively, with SE = 20.211 and 18.452), t(12) < 1.

## Experiment 3: weight reproduction

### Participants

### Stimuli and apparatus

^{°}turns with their upper body to fill the empty bowls.

### Design and procedure

### Results

^{2}= 0.36. Figure 5 shows that our 60 left-to-right counters produced lighter weights with the left compared to the right bowl (M =137.85 g vs. 152.467 g, respectively, with SE = 43.136 and 3.211), t(59) = 8.79, p = 0.001, Cohen’s d = 0.52. In contrast, the 44 right-to-left counters produced heavier weights with the left than the right bowl (M =150.212 vs. 145.318 g, respectively, with SE = 3.662 and 3.750), t(43) = 2.51, p = 0.016, Cohen’s d = 0.16.