From what I can gather based on the article cited below, left-branching structures are more accessible than right-branching ones to speakers with the opposite branching direction. This is the opposite result of what I expected; I thought right-branching languages would just be easier to parse across the board, but not by a significant amount.

The article points out that users of right-branching languages can make parsing decisions immediately and users of left-branching languages must delay these decisions -- or make speculative decisions using statistical or semantic information.

If there is indeed a difference between the strategies used by left-branching and right-branching language users for parsing, or a difference in the extent to which strategies from a common pool are used, then it is possible that the strategies favored by left-branching speakers will not carry over as well to a novel language. Or they might carry over better.

This article, published in Nature, is about an experiment exploring the differences in working memory performance for initial and final stimuli in speakers of left and right-branching languages. It contains the following remark as part of its background material before the experiment.

Speakers seem to develop a "bias" toward the branching direction more common in their language, so that LB structures are harder to process for RB speakers (due to the higher working memory needed to retain the intermediate products of computation 67,68,69; see 70 for experimental evidence), but they are more accessible than RB structures for LB speakers 65, 71, 72.

If I'm interpreting the above sentence correctly, this means that speakers of right-branching languages are better able to interpret left-branching structures than left-branching

Here are the references cited in that sentence (and 66 because bulleted lists are forced to be numbered consecutively).

65. Mazuka, R. & Lust, B. In Proceedings of NELS 18 (eds Blevins, J., Cart, J.), 333–356 (Univ. of Massachusetts, Amherst, 1988).

66. Pienemann, M. (ed.) Cross-linguistic aspects of Processability Theory (John Benjamins Publishing CO, Amsterdam, 2005).

67. Frazier, L. & Fodor, J. A. The sausage machine: a new two-stage parsing model. Cognition 6, 291–325 (1978).

68. Gibson, E. Linguistic complexity: locality of syntactic dependencies. Cognition 68, 1–76 (1998).

69. Kemper, S. & Kliegl, R. (eds) Constraints on language: aging, grammar, and memory (Kluwer Academic Publishers, Boston, 2002).

70. Friederici, A. D., Chomsky, N., Berwick, R. C., Moro, A. & Bolhuis, J. J. Language, mind and brain. Nat. Hum. Behav (2017).

71. Lust, B. & Mazuka, R. Cross-linguistic studies of directionality in first language acquisition: response to O’Grady, Suzuki-Wei and Cho, 1986. J. Child Lang. 16, 665–684 (1989).

72. Lust, B. (ed.) Studies in the acquisition of anaphora (Kluwer, Boston, 1986).

From what I can gather based on the article cited below, left-branching structures are more accessible than right-branching ones to speakers with the opposite branching direction. This is the opposite result of what I expected; I thought right-branching languages would just be easier to parse across the board, but not by a significant amount.

The article points out that users of right-branching languages can make parsing decisions immediately and users of left-branching languages must delay these decisions -- or make speculative decisions using statistical or semantic information.

If there is indeed a difference between the strategies used by left-branching and right-branching language users for parsing, or a difference in the extent to which strategies from a common pool are used, then it is possible that the strategies favored by left-branching speakers will not carry over as well to a novel language. Or they might carry over better.

This article, published in Nature, is about an experiment exploring the differences in working memory performance for initial and final stimuli in speakers of left and right-branching languages. It contains the following remark as part of its background material before the experiment.

Speakers seem to develop a "bias" toward the branching direction more common in their language, so that LB structures are harder to process for RB speakers (due to the higher working memory needed to retain the intermediate products of computation 67,68,69; see 70 for experimental evidence), but they are more accessible than RB structures for LB speakers 65, 71, 72.

If I'm interpreting the above sentence correctly, this means that speakers of right-branching languages are better able to interpret left-branching structures than vice versa.

Here are the references cited in that sentence (and 66 because bulleted lists are forced to be numbered consecutively).

65. Mazuka, R. & Lust, B. In Proceedings of NELS 18 (eds Blevins, J., Cart, J.), 333–356 (Univ. of Massachusetts, Amherst, 1988).

66. Pienemann, M. (ed.) Cross-linguistic aspects of Processability Theory (John Benjamins Publishing CO, Amsterdam, 2005).

67. Frazier, L. & Fodor, J. A. The sausage machine: a new two-stage parsing model. Cognition 6, 291–325 (1978).

68. Gibson, E. Linguistic complexity: locality of syntactic dependencies. Cognition 68, 1–76 (1998).

69. Kemper, S. & Kliegl, R. (eds) Constraints on language: aging, grammar, and memory (Kluwer Academic Publishers, Boston, 2002).

70. Friederici, A. D., Chomsky, N., Berwick, R. C., Moro, A. & Bolhuis, J. J. Language, mind and brain. Nat. Hum. Behav (2017).

71. Lust, B. & Mazuka, R. Cross-linguistic studies of directionality in first language acquisition: response to O’Grady, Suzuki-Wei and Cho, 1986. J. Child Lang. 16, 665–684 (1989).

72. Lust, B. (ed.) Studies in the acquisition of anaphora (Kluwer, Boston, 1986).